I used to think that if a movie either had flashbacks to an adults childhood, or they started when they were a child or anything that required an adult character to be shown as a child, or vice versa, that they filmed the parts in childhood when the actors were that age, and then they had to wait for the actors to grow up to be the right age and film the rest of the movie.
A childhood belief about films

others believed things like this
- I used to beleive that in order to rewind a film the actors would have to re-act the whole movie backwards. — movies backward
- i used to belive that actors in movie used to really get killed. Until i saw an actor die twice— calipzo avery
- I used to think that at the movie theatres, the actors were just acting behind the screen. — Srushti
- When a grown-up is shown as a child, it was actually them, and the film took 20+ years to produceAnon
- As a child I used to think that actors really died in movies.— Jordi Comendador
- I used to believe that EVERYTHING we did was being filmed for someone somewhere to watch as a movie— KatBird_27