If you are narrating, ie speaking whilst the movie is also playing you need to use the present tense. If you are telling someone what happened in the movie AFTER watching it you use the past tense.

You can use either tense , but it is very important that you use the same tense throught the film. I would rather use the present tense because it give a feeling of immediacy , and then if they use "flashbacks" there will be less confusion as you describe those events.