Top 5 Roles?

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Moon
Better Living Through Chemistry
Mr Right
The Way Way Back
Three Billboards

Moon
Seven Psychopaths
Three Billboards Outside Ebbing, Missouri
Galaxy Quest
Confessions of a Dangerous Mind

The Green Mile
Charlie's Angels
Confessions of a Dangerous Mind
Matchstick Men
Seven Psychopaths
(in no particular order)

But a character actor of his type can't really be pinned down to only 5 performances because he brings different elements of his talents in so many of his roles that it's really just a matter of personal taste

I would say that the one movie that MUST be on any top 5 list among his fans would be Confessions of a Dangerous Mind, if only because it allows him to play the lead and being the lead in a Charlie Kaufman script means that he has so much room to show his full talent

Rockwell could have been a leading man in some other universe. He just happens to not be one because the leading man casting category is so crowded. But he's always charismatic, he's almost always sleazy and clever and less noble than most of the other characters that surround him