The chance is high that when you are using a movie in your presentation, it is a fragment of a longer feature film that you cut out using iMovie or another movie editing software. Make sure to create a seamless transition between the movie fragment and the presentation.
The start. Someone who is watching the full length movie has gone through the process of transitioning into the movie story. Usually the director will take you there slowly, step by step. The presentation audience lacks that context. Landing them straight into that massive shootout scene at minute 14:49 will create confusion, and requires 30 seconds or so to grasp what is going on, and during that time they are probably not paying attention to that critical punch line that you want them to hear.
What can you do? Carefully select your cover image of the movie. It does not have to be the first scene, it can be a frame that comes later in the movie, or even a frame that is not part of the scene you selected. Do not set the movie to autoplay, but take a few seconds to tell the audience "where they are going next".
The end. A cut out movie fragment often has an abrupt ending. Again, dampen the transition by putting another image at the end of the sequence that gives you the opportunity to lead the audience back into the story of your slides.