Unless instructed otherwise, once a movie starts, it plays through every frame in the Timeline. Use the Play and Stop statements to stop or start a movie at specific intervals. For example, you can stop a movie at the end of a scene before proceeding to the next scene. Once stopped, a movie must be explicitly started again. You must use the Play statement to restart the movie.
Play and Stop are most commonly used to control movie clips with buttons. To do so, however, the movie clip must have an instance name and must be targeted with Tell Target. See Controlling other movies and movie clips.