don't wait for her to make the first move... you have to just go for it. You'll know when the time is right. (I'd say sometime after the movie)
also, having your arm around her in the theatre isn't very comfortable... it's an awkward and restricting sort of contact. Stick to making contact with your hands or legs.
and try not to overthink this (cause it sure seems that way). Just let things happen naturally and you'll both be a lot happier for it
