He always seemed like an actor who actually was nice, rather than one who was good at acting nice. Comments from people who knew him seem to confirm that. And he was a bloody good actor.
"Just as talented as Sean — just a lot less cocky," Slate magazine critic Cintra Wilson wrote of him last year. He "makes you seamlessly believe in characters so much you barely even notice them."
Just as talented? Far more so, I'd say. If only Sean Penn were anywhere near as talented as his brother, his films wouldn't be so irritatingly tedious. Cintra Wilson's right: truly great acting is something you don't even notice. That's the whole point. The moment you notice the acting, the acting has failed. That's something Sean Penn has never understood. Chris was a master.
(While we're on the subject, Tom Cruise blew Dustin Hoffman off the screen in Rain Man. Same goes for Ethan Hawke, Denzel Washington, and Training Day.)