A bit part is a supporting acting role with at least one line of dialogue (a supporting role with no dialogue is called a walk-on). In British television, bit parts are referred to as under sixes (fewer than six spoken lines). An actor who regularly performs in bit roles, either as a hobby or to earn a living, is referred to as a bit player, a term also used to describe an aspiring actor who has not yet broken into major supporting or leading roles.

Unlike extras, who do not speak any lines at all, actors in bit parts are typically listed in the credits. An exception to this practice is the cameo appearance, wherein a well-known actor (or other celebrity) appears in a bit part; it is common for such appearances to be uncredited. Another exception occurred in MGM's 1951 screen version of the famed musical Show Boat, in which the role of the cook Queenie (Frances E. Williams) has been reduced from a significant supporting role in the stage version to literally a bit part in the film. Ms. Williams, whose appearance was not intended as a cameo, was not listed at all in the credits. On the other hand, William Warfield, whose role as Joe, Queenie's husband, was also drastically shortened in the film from the stage original, did receive screen credit because he sang Ol' Man River.

Bit parts are often significant in the story line, sometimes pivotal, as in Jack Albertson's role as a postal worker in the 1947 feature film Miracle on 34th Street. Some characters with bit parts become well remembered. A good example is Boba Fett, with very few lines in The Empire Strikes Back and none (except a scream) in Star Wars Episode VI: Return of the Jedi. Constantin Stanislavski famously remarked that "there are no small parts, only small actors".

Dabbs Greer, a famous bit actor, once said, "Every character actor, in their own little sphere, is the lead. "