Don’t forget about Bryan Cranston either! He was that weird but hilarious dentist from Seinfeld and the dad in Malcolm in the Middle way before he ever was Walter White.
Man, Robin Williams can get to me like no one else. There were moments in Good Morning Vietnam, a couple years before Dead poets, but GMV was def more focused on his big comedic personality. Then his performance in The Fisher King...I don't generally get emotional about celebrities but that dude was different.
There's been a few musicians that have been much better than expected. Dolly Paton in 9 to 5, Madonna on A League of their Own, Prince's acting in Purple rain. Ice Cube in Boyz in da hood.
I figure it's also because they're on stage so much and trying to convey so much to an audience so often. There's gotta be a lot of transferrable skills.