The very first scene writer Emerald Fennell thought of was a female character lying on a bed while someone takes her pants down, as she is drunkenly asking "What are you doing?", then asking the same question in a completely sober manner. She wrote the rest of the film based on this idea, and the full scene did end up in the finished film.
The title is likely a reference to Brock Turner, a Stanford University student who was convicted of sexual assault in 2016. The judge sentenced him to only six months in prison on the basis that he was a "promising young man."
In an interview, Emerald Fennell said that Carey Mulligan was very nervous about having to act drunk, a notoriously difficult thing to do. Fennell reminded her that she wasn't actually acting drunk, but rather was playing a character that was sober, pretending to be drunk, so it didn't need to be perfect. This helped Mulligan relax, and Fennell was quick to point out that her drunk acting was very good.
The words "rape" and "sexual assault" are not spoken in the film.
Emerald Fennell said that one advantage to filming in L.A. was that production was able to entice a lot of her first choice actors to sign on for the smaller roles in the film, since it involved little to no travel and a much shorter time commitment on their part. She referred to the cast as a whole as "an embarrassment of riches."