Yes, you are free to copy the individual frames or images from a public domain movie without permission. Film elements such as music or promotional photos are also usually public domain as well although, on rare occasions, they may be under separate copyright.
What about movie stars? You are free to copy a movie star's image from a public domain movie without permission (and use it for informational purposes). You need to be careful in one particular instance -- when you are using the movie star for a commercial purpose (that is to endorse a product). That's because all living celebrities and some deceased ones have a right of publicity that permits them exclusive rights for endorsement purposes.