Yeah, the main thing with girls is BE REAL. Don't do anything you wouldn't normally do, don't act strange or try to be "Mr. Cool." Just be yourself. The thing girls appreciate more than how "Manly" or "Cool" you are is your feelings towards them. Don't let anything stupid like pride get in the way. If you don't want to spill it right there, just be calm (And yes, I know how hard it is.) and ask her to that movie.
Also, take Kinjo's advice, BUT -- Remind her before she leaves. Make it friendly, like "I had a great time tonight. Go home safe, and I'll see you at X Movie on X Day?" Something like that.