JAL has previously discussed a lot of this in interviews, posts on this forum (like this one), and afterwords, especially the afterword to the 2015 Swedish paperback edition of LTROI (which can also be read on his website). Summarizing all of what JAL has ever said about LTROI is beyond the scope of this post, but I'll reference them when they provide additional details on things mentioned here.
To set the context, I'll quickly recap the story of how JAL got the idea to write the novel. The following is from the 2015 afterword.
Before becoming an author, John worked as a stand-up comedian and wrote humor monologues and TV sketches, but over time the job offers got fewer and fewer. He tried writing novels and plays in different genres, but none of them turned out very good since, according to him, he tried too hard at being a "serious" author. In late 2000, his wife Mia Ajvide, who at the time worked as a high school teacher, was going to have a horror project with her students and borrowed some books about the genre's history. This was in turn inspired by the fact that John had convinced her to watch a bunch of horror films, since he had been a fan of the genre since his early teens. John read the books Mia borrowed and got the idea to try writing a horror story of his own just to see if he could. Over the course of a few days he wrote the zombie short story Våran hud, vårat blod, våra ben ("Our skin, our blood, our bones", after a line from a Morrissey song) and for the first time felt like it was easy and something he knew how to write. Encouraged by the success, he started thinking about doing an entire novel in the same genre.
Side note: Våran hud, vårat blod, våra ben wasn't published until the 2015 edition of Pappersväggar, in which it was renamed to Kommunion ("Communion"); JAL reused the original title for a Swedish short story anthology which came out in 2016. Kommunion has never been released in English as far as I know, but in my opinion it's a pretty unremarkable story and Tjärven is basically a bigger and better version of it, so you're not missing much.
LTROI's working title was Den ende vännen ("The only friend"), a name JAL was not at all satisfied with. "Låt den rätte glida in ("Let the right one slip in", from the Morrissey song) was originally only meant to be used as a chapter name, specifically for the part where Oskar and Eli's relationship deepens. JAL changed the book's title to Låt den rätte komma in shortly before sending it to potential publishers in the winter of 2001.
On another note, current-JAL mentions in the Little Star chapter that he at one point intended to name one of LTROI's chapters "Ett tillgivet djur" ("An affectionate animal"), in reference to a line from Björn Afzelius' song Ikaros.
- December 11, 2000: JAL's first, basic idea was to have something horrible come to Blackeberg and then see what happened. He chose a vampire partially because that meant he got a bunch of elements for free, since most people already know what a vampire is. The earliest notes for the novel read:
A vampire novel. That takes place in Blackeberg. I'm not sure that it's about vampires specifically, but something similar.
We have a courtyard resembling mine. In a suburb. A lonely child. An outcast. Then a new boy moves into the yard and becomes his friend. The new boy is a vampire. The dad or mom the vampire lives with is only a helper. A normal human under his control. Maybe the boy only comes out at night, during which he sucks blood from children. The risk is that everything becomes too macabre. I don't know if the people he bites become vampires themselves. In that case, maybe they hide in the cellar offices. I want a scene in the bathhouse. This clean, chlorine-smelling place.
- Like most here probably already know, the bathtub of blood is inspired by the blood-filled coffin in Carmilla. The thinking behind it was that Eli breathes blood (something JAL also mentions on the UK commentary track for the movie); more specifically, that he collects the stuff by sucking it from his victims and then releases it out into the tub as a form of metabolism.
- January 2, 2001: John decided to set the story during winter after he had a dream in which he ran across a field and sank into snow, which made him wonder if the vampire would die the same way.
- January 22: JAL knew almost from day one that he wanted the book's climax to consist of a a massacre in a bathhouse, but during a bathhouse visit with his son, he decided that an aimless slaughter among children would be too much and that the victims would instead be three bullies tormenting the main character. He considered having a scene where the bullies would throw Eli into a hole in the ground and tie Oskar to a nearby tree with the intent of making him stand and watch as Eli burned to death when dawn arrived.
- JAL also considered including a "hero": another boy, not one of the bullies, who wanted to kill the vampire. He figured this would create an interesting dynamic in Oskar, who would both be the sympathetic bullying victim and one of the bad guys the hero had to fight, but this dynamic ended up becoming so interesting that the need for a hero was lost. Lacke was added later and took over several of the hero's narrative functions.
- Gösta's cats originally had another role. JAL envisioned a scene where a group of children would decide that the monster hunting them was "the Witch" (an old lady with cats in a stinking apartment, based on a woman John encountered as a child) and kill her. Her cats would run loose throughout the rest of the book.
- JAL initially pictured a growing vampire epidemic as an important part of the plot, with more and more children and teens becoming infected. Current-JAL explains that he initially didn't trust his own ability to make the story exciting without a constant escalation of events. One scene after the epidemic had gotten going would've involved "the Fool", a "harmless pedophile" whom the infected children would attack and drain with "kisses" until he died in ecstasy, inspired by an old comic cover showing Batman getting attacked by vampire children. The idea was modified and reused for Max Hansen's death in Little Star.
- February 11: The notes' first mention of the main characters' names and Eli's backstory:
The boy is named Oskar. The vampire is named Eli. You don't know if it's a boy or girl name, and the vampire is androgynous. I imagine that he's a castrated boy. Let's say that he was in some nobleman's court long ago, got mutilated into a castrato singer. Or that the nobleman was a vampire who took Eli's blood by cutting off his penis, but Eli was infected with vampire blood in the process.
- February 14: A note from this date describes Eli as 400 years old instead of 200, meaning he was born in the 1560s instead of the 1760s in early drafts.
- "The Helper" (Håkan) was a completely different character in early versions: a very attractive, sexually perverse man who killed for Eli in exchange for getting to drink his blood. The idea was that vampire blood is much stronger than normal blood, and drinking it gave the Helper the strength and charisma to seduce any woman he wanted. This version of the character created many problems since, current-JAL says, he can't describe the thoughts of someone he doesn't believe in or empathize with on at least some level, so on February 16 it got replaced with the Håkan we know now:
The Helper is a big problem. The part with his night on the town is stone dead. [...] I don't feel for him. And on some level I have to. If not as a human, then as a novel character. [...] He shouldn't be as handsome or easygoing as I've made him. He's more Renfield, longing for his Master's reward. He loves Eli. Maybe he's a pedophile. [...] Let's say that he's secretly filmed children in a shower at some point. Has never assaulted a child. He's the struggling pedophile. This urge that has to be suppressed has made him a heavy alcoholic. Eli has pulled him out of the gutter. He's newer in Eli's service than what has been said. He has only committed two murders before. He hates it. He cries while packing the bag of tools. But he does all this for the sake of love. So that what he loves most will survive.
There we go. Now we have a real tragic figure. The money he gets from Eli he'll distribute among the poor, or anything. To atone for his sins. [...] This also makes Eli significantly scarier.
The Helper is sitting in prison with his melted face and waiting for Eli to come and free him. Eli instead [sends a Hells Angels member to kill him]. I can see the headlines with his Quasimodo face. The Monster. People's hatred of him. The joy when he is killed. He who did all this for love. Heavy stuff, and now hot damn am I starting to feel for him. [...] Now I'm starting to fear Eli. That's good. It's about time.
- The policeman Gunnar Holmberg was a major character in the first draft. His wife would have a private interest in forensics, specifically bloodstain pattern analysis (which JAL had read a couple books about), and the two would have domestic conversations where they discussed "mysterious" clues and connections between different murder cases. At one point (March 9), JAL also considered depicting the bathhouse massacre through a forensic reconstruction after the fact where Eli's abilities would only be hinted at to the reader (like describing how a drop of blood had fallen from a height that would only be possible if the victim hovered in the air). A faint echo of this remains at the end of the novel during the police investigation of the pool. On May 19, JAL decided to reduce Gunnar to a minor side character after he realized that Lacke had essentially taken over his role in the book. Current-JAL describes Gunnar and his wife as a "far too lovely couple for this story". Stefan and Karin from Let the Old Dreams Die are a modified version of them.
- March 14: JAL mused that while Oskar is locked into roles by others (Piggy, Sweetheart), Eli consciously becomes different roles for different people (the helpless child, the loved one, Elias), and that solitude, among other things, is that you define yourself and are no longer defined by others. JAL considered portraying Eli as not having a reflection, as a metaphor for how he with time has become "an everything and a nothing" who is never reflected in anything but himself. One idea was that Oskar would get the definitive proof of Eli's vampirism in a scene where they would be lying on a bridge and Oskar would notice that Eli wasn't reflected in the water. However, he ultimately decided he didn't want to go that far in the supernatural direction.
- March 26: Apparently Lacke originally had or was in the process of getting a PhD thesis. This was removed because it added a tad too much eccentricity to the already-intense story.
- The part of the story that presented the biggest challenge for JAL was also the most important one: the relationship between Eli and Oskar. He rewrote the two's early scenes more times than anything else from any of his books. The problem was quite simply: "Why does Eli care about Oskar?" (John has talked about this several times, so the following is a summary of what's said in Misslyckas igen, the 2015 afterword, and a 2019 interview with the podcast Udda Ting.) In early versions, JAL made Eli too "badass"; a proud, wise, and tragic figure, roughly the sort of supernatural friend John himself fantasized about when he was a bullied 12-year-old. When he read the Oskar/Eli scenes aloud to Mia, she kept pointing out that while it made total sense for Oskar to be fascinated with Eli, there was no reason for Eli to return the feelings since he stood so far above Oskar. It took a lot of rewrites back-and-forth and persistence from Mia—with grumpiness from John as a result—over the course of several months (the problem is brought up repeatedly in the notes, from April 30 to July 14) until John had sufficiently weakened Eli and put him on a level matching Oskar's, made him someone who is in essence a lonely and damaged child who hasn't gotten the chance to be a child in many years and longs for a playmate. This process also involved letting go of the normal vampire tropes to examine what a child vampire's gross and depressing existence would actually look like. JAL emphasizes that the success of the book and its adaptations all hinge on the early Oskar/Eli scenes, and if it wasn't for Mia's persistence he probably wouldn't have the career he does now.
- April 30: Eli was originally supposed to have extensive shapeshifting abilities. He got access to Tommy's cellar office by taking on the appearance of an attractive woman from one of Tommy's magazines. In order to grow in length and volume he had to carve out flesh from his back like a skogsrå. JAL describes it as a very funny scene to write, but it didn't fit the story as it would've made Eli too powerful.
- In order to better depict Eli's ambivalence and desire to be a child again, John thought about adding an early scene where Eli would ride snow racers with Oskar and react with strange sadness, "like an adult repeating games from their childhood". He also wanted them to go into the ghost house together.
- May 7: One idea was that Gösta would accidentally get infected with Virginia's blood during the cat attack scene. When Lacke and the others came back to visit Gösta a few days later, he would've killed and drained the blood from every cat in his apartment. Current-JAL says that he has no idea why he left out this "glorious" scene and that he might just have forgotten about it.
- May 8: The squirrel scene was a private joke about ekorrbilder (literally "squirrel pictures"), a Swedish term for neutral nature shots in movies.
- Despite how iconic they are, the Rubik's Cube and the scene where Eli bleeds were both inserted fairly late in the process (May 8 and August 5, respectively). Also, before deciding to include the bleeding, JAL pictured a scene where Eli would demonstrate his abilities by transforming his face into a copy of Oskar's, which Oskar would then kiss. It seems JAL hadn't completely abandoned the idea of a shapeshifting Eli at that stage.
- August 29: Like John has said elsewhere, he was completely shaken after writing the scene with Håkan and Eli in the cellar, more than anything else he's written before or since. It was intended to represent all the abuse Eli had been subjected to his life and thereby balance the act of violence Eli himself commits in the bathhouse, but the original version was even worse—specifically, Håkan's rape attempt didn't end up just being an attempt—and it's the only instance where his publishers at Ordfront have told him to rewrite a scene because it was too horrific.