How could i help my girlfriend overcome the fear of escalators caused by a movie we saw ?

Your best bet would be to get her on an escalator. But do not by any means try to force her onto one without her consent, this will most likely reinforce her fear of them and make the phobia worse. You can really only help her if she wants the help. If she does, establish your trust with her (you're dating her, that shouldn't be an issue), reure her that this is a healthy step for her to take (reducing her fear), and stick with her every step of the way, maybe even let her ride piggy back up an escalator a few times if she'll agree to that before she'll get on on her own. Just be very gentle and very comforting. The more at ease you can put her, the easier the whole process will be.