I'd like to know if I need to add "the" before "movie stars".
A/The teacher asked his/her students to give a speech.
Teacher: Please describe movie stars you know.
Note my corrections above. It's a strange thing for the teacher to say. How many students actually know a movie star? Perhaps the teacher should have used "know of", "know about", "are familiar with" or "like". How many movie stars was each student supposed to talk about? The instruction suggests that each speech should involve more than one movie star.
I have a feeling the teacher's instruction would have been clearer as "Please tell us about your favourite movie star".