The wallpaper on Lara Jean's phone showing her and Peter asleep was actually the actors relaxing together off-set. A crew member snapped the picture of them and used it in the film.
Noah Centineo improvised the spin scene in the cafeteria, where Peter enters with his hand in Lara Jean's back pocket (suggested by Lara Jean in the movie, and Peter himself in the book).
The letters seen in the movie, including the contract, were all physically written by Lana Condor. During a lunch break Lana had to write several copies as her character Lara Jean rips them.
The letter to Josh reads: "Josh Sanderson, I liked you first. By all rights, you were mine. And if it had been me, I'd have packed you in my suitcase and taken you with me, or, you know what, I would have stayed. I would have never left you. Not in a million years, not for anything.
P.S. I still love you and that's a really huge problem for me and it's also a really huge surprise. I swear I didn't know. All this time, I thought I was over it. How could I not be, when it's Margot you love? It's always been Margot."
Jenny Han, the writer of the books, mentioned in an interview that loads of studios were interested in adapting the book into a feature film, with only one condition: change the race heritage of Lara Jean to Caucasian. Han, determined to keep Lara Jean's heritage as half Korean, half American, said only one studio agreed with her choice and cast an Asian actress as Lara Jean. Lana Condor, who is an adoptee in real life, was the result. However, Jenny Han did not insist any of the boys be cast as Asian. As a result, the cast of boys Lara loved consists of only four Whites, one Black, and none of the boys Lara loved are Asian.
Jenny Han: The author of the novel of the film, as a chaperon in the scene where Lara Jean and Lucas dance together.