A performer owns copyright of their performance, unless otherwise agreed.
No, not quite.
A mere performance, by itself, is not copyrightable. In order to be copyrightable, a performance must be fixed in a tangible medium. This always raises the question of whether the person doing the fixation is the actual author, or at least a joint author, with equal rights in the work. Basically it hinges on creativity. If the actor is in charge of their own costuming, lighting, cinematography, and direction, and everyone else is just following orders like a robot, with no creative input, and we set aside issues of works made for hire, then yes, the actor would be the sole author of the film. But if the actor isn't in charge of everything which, along with the performance, is being filmed, then they may be only one of many authors, and if it's the actor who is following orders like a robot, the actor may not have contributed any sort of authorship at all.
Burrow-Giles Lithographic Co. v. Sarony is what you'll want to take a look at.