Don't show your intentions just yet. Think of it as getting to know her to decide if YOU want her. Unless you are getting very clear signals, you need to build some sexual tension before you make out, or things are going get awkward FAST. A movie, sadly, is not necessarily the best venue for this.
You need to start the night off by getting her laughing. Playful teasing, LOTS of kino right of the bat--or else it'll seem creepy later. You don't want to be the guy that stretches and then puts his arm over a girl during the movie.
Get a snack that you can share before the movie (big thing of popcorn, etc.). Get her to giggle when the lights go out and give her a loud "Shhhh!" If she's not giggling, that's not a good sign.
If there's a point in the movie that's scary, or even just mock-dramatic, tell her half joking to give you her hand and you'll get her through it. Start to kino again and escalate if she seems receptive. If she's giving you those eyes and holding your gaze, she's ready.