Actors often have to make an effort to change their physical features in order to truly embody their characters. Though some may not make extreme life- and health-altering choices like Robert De Niro or Christian Bale, they are willing to change their outward appearance. Bald actors wear wigs - even if they are noticably terrible ones - while those with full heads of hair shave their heads. But what if that's too much to ask of an actor?
Not to worry! For those who don't want to risk their luscious locks, a production's makeup department can instead glue a a piece of vinyl onto the actor's head and blend with cosmetics to make an inauthentic chrome dome.
Obviously, bald caps in movies and tv do have their problems: they send a reasonable normal looking person down into the dark abyss of the uncanny valley. Their overly oblong heads, their emotionless and unmoving upper brow, and the obvious line between their vinyl and human flesh set these actors apart from their naturally bald counterparts. But for some actors, it's better to look like a space alien dressed as a human in a film that will continue to be watched long after they've retired, than suffer having no hair for a few months.