Performances that steal the show...

I can’t believe that no one’s said this already: Rutgar Hauer in Blade Runner. He MADE the movie. Caveat: the OP specified someone who stole the show “from the [singular] main character.” The main character means the protagonist, I believe. Hauer was a brilliant, complex antagonist, a feature the Blade Runner sequel sorely lacked.

Also, Scott Glenn in Urban Cowboy. In her critique of the film in The New Yorker, Pauline Karl wrote that “Glenn looks like he’s about to ejaculate bullets.”

Dennis Farina in Get Shorty. He played a fantastic thespian rhythm guitar to Travolta’s and Hackman’s and DeVito’s leads. He was the mortar that held the acting bricks together.

And thank you, sunny day. I just now read your post, hence this edit.

Alec Baldwin in Glengarry Glen Ross. I know he's only in one scene but its really the only part of that movie that I remember.

I would also say Gary Oldman in both True Romance and The Fifth Element.

Some might not like this, but...

Harrison Ford in Star Wars

Not the lead but Brian Dennehy in The Next Three Days had enormous presence even though he didn't speak until the end of the film.

De Niro in Jackie Brown

Charles Grodin in Midnight Run (1988)

jerry lewis in the king of comedy