Situation 1: Just sort of let your hand drop next to hers, and try to “accidentally” brush her hand, and see what she does.

Situation 2: Put your arm in the same, or one next to yours, armrest. just brush her hand, and see what she does.

Suggestion: If it is a romantic scene in the movie, see if she’s looking at you, but look without her noticing! If she is, then this will be easier. Just put your hand on top of hers either way.