My son has Asperger's and I have taken him to the movies and other events. Depending on the severity of autism, explaining exactly what to expect and maybe letting him pick the movie after watching the trailer is helpful.
Generally movies are very loud so I make sure I have ear muffs to dub the sound. We also sit in the way back so there is distance from the screen and he has a chance to absorb what's going on - instead of feeling like he's IN the movie.
Afterwards we talk about the movie. He will tell me he likes it or doesn't. I've only had to leave the theater ONCE and the theater manager was kind enough to refund my money because we were only there for 5 minutes. Good luck to you.