A scene stealer is a character in a film

Film

A film, also called a movie or motion picture, is a series of still or moving images. It is produced by recording photographic images with cameras, or by creating images using animation techniques or visual effects...

 or drama

Drama

Drama is the specific mode of fiction represented in performance. The term comes from a Greek word meaning "action" , which is derived from "to do","to act" . The enactment of drama in theatre, performed by actors on a stage before an audience, presupposes collaborative modes of production and a...

tic performance that dominates the audience's attention, often through charisma

Charisma

The term charisma has two senses: 1) compelling attractiveness or charm that can inspire devotion in others, 2) a divinely conferred power or talent. For some theological usages the term is rendered charism, with a meaning the same as sense 2...

, humour

Humour

Humour or humor is the tendency of particular cognitive experiences to provoke laughter and provide amusement...

 or powerful acting, thus "stealing the scene" or "stealing the show". The term usually (but not always) applies to a supporting character, such a sidekick

Sidekick

A sidekick is a close companion who is generally regarded as subordinate to the one he accompanies. Some well-known fictional sidekicks are Don Quixote's Sancho Panza, Sherlock Holmes' Doctor Watson, The Lone Ranger's Tonto, The Green Hornet's Kato and Batman's Robin.-Origins:The origin of the...

, best friend, confidant

Confidant

The confidant is a character in a story that the lead character confides in and trusts. Typically, these consist of the best friend, relative, doctor or boss.- Role :...

, villain

Villain

A villain is an "evil" character in a story, whether a historical narrative or, especially, a work of fiction. The villain usually is the antagonist, the character who tends to have a negative effect on other characters...

, etc. Sometimes the term is used for a minor character who has unexpectedly (and perhaps inappropriately) drawn attention away from the star

Leading actor

A leading actor, leading actress, star, or simply lead, plays the role of the protagonist in a film or play. The word lead may also refer to the largest role in the piece and leading actor may refer to a person who typically plays such parts or an actor with a respected body of work...

. See McLovin.