During filming, the cast gained access to home movies, photographs, and recollections from Steven Spielberg's family's past to learn what they were like and how to portray the fictionalized versions of them (The Fabelman family) on screen, while making them feel fresh and original. Paul Dano reflected: "It was overwhelming and it was sort of a heavy cloak to bear because we were with someone who was having a big experience everyday, revisiting and reworking through a part of their life...For somebody like Steven to share that much of himself with us---with the audience too---it was really a profound experience."
Steven Spielberg said his parents had been "nagging" him to put them on the big screen prior to their deaths. "They were actually nagging me, 'When are you going to tell that story about our family, Steve?' And so this was something they were very enthusiastic about," he said. He also shared what finally prompted him to make The Fabelmans: "I started seriously thinking, if I had to make one movie I haven't made yet, something that I really want to do on a very personally atomic level, what would that be? And there was only one story I really wanted to tell." He also said The Fabelmans is "the first coming-of-age story I've ever told." "My life with my mom and dad taught me a lesson, which I hope this film in a small way imparts," he told The Hollywood Reporter. "Which is, when does a young person in a family start to see his parents as human beings? In my case, because of what happened between the ages of 7 and 18, I started to appreciate my mom and dad not as parents but as real people."
Seth Rogen told reporters that Steven Spielberg often became emotional on the set during production. "It was a very emotional experience. He was crying a lot on set," he said. "It's very directly based on his life and pretty much everything that happens in the movie is something that happened to him. As we were shooting, I'd be like, 'Did this happen in real life?' and the answer was 'yes' a hundred percent of the time."
For the scenes of Sammy filming his own 8mm films, Steven Spielberg decided to have the character re-create the exact ones he made during his childhood, and worked with cinematographer Janusz Kaminski to ensure that they were portrayed as accurately as possible, but with improvements in the camera angles. A few of the films portrayed in this film include the Western short film that he made with his fellow Boy Scout friends in order to earn the photography merit badge, the war short film Escape to Nowhere (1961), in which he used his school bully by casting him in the lead role in an effort to face his fears, and Spielberg's re-creation of the train wreck scene from Cecil B. DeMille's The Greatest Show on Earth (1952). Spielberg remarked, "It was joyful being able to recreate those films....I shot a lot of films when I was a kid on 8mm. It was unique in those days. Not a lot of people were going out and shooting in 8mm. It was physical; it was a craft. You had to sit there with a...splicer, and then you had to scrape the emulsion off the film in order to get a seal so when you put glue on it, you literally glued the film together. And I must say, I miss it."
Steven Spielberg claimed that although he and Tony Kushner had discussed the idea of the movie for years, it was during the COVID-19 pandemic in 2020 that he decided to write the screenplay with Kushner from scratch while his schedule was clear. The two of them wrote the script from their homes during lockdown and finished it in two months.