Turn off watches, and phones before entering the theatre.
Take off coats, sweaters, etc. before you walk to your seat, not after you sit down.
In a non-stadium type theatre, if there are other choices available, don’t sit directly in front of anyone, especially if you’re very tall, or have some crazy big hairstyle. If you must sit directly in front of someone, have the courtesy to sit low and ask if they can see past you. Don’t wear a hat.
If the theatre is full, don’t sit in an empty seat without asking if it is taken. An article of clothing or property on an empty seat generally indicates that someone has claimed that seat. Move on and look elsewhere.
When you need to climb over people to get to your seats, politely acknowledge their presence and apologize. If someone stands up to let you by, thank them. Be very careful moving past other people in a row to get to your seat. Don’t step on anyone’s feet or knock over their snacks. Don’t spill your snacks on anyone.
And speaking of snacks, open candy and snack wrappers before the movie starts!