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.
For those not in the know, Ermey was only ever meant to be a consultant to teach Tim Colceri, the actor playing the drill instructor, how to do so realistically. It was only after Ermey did a 30 minute demonstration in which he berated and insulted the "troops" of the film while stagehands beaned him with tennis balls and oranges that Kubrick realized that casting Ermey would essentially just be removing the middleman.
Tricia Helfer comes to mind first. In Battlestar galactica. I forget the story but it’s something like she was brought in for her looks(they wanted a knockout for the first scene cylon). Then she went on to blow them away with her acting. She was a model before if I recall.
Eminem in 8 Mile. When I first saw it I was convinced he'd become a superstar actor and stop making music, but that never happened. Even so, he's fucking phenomenal in that movie.
Dominique Tipper in The Expanse (E: yeah, not a movie, but still an awesome experience all around)
I mean, not everyone in the community thinks much of her acting, but on Season 5 she puts out a performance that puts a lot of veteran actors to shame.
Andy Buckley, the guy who played David Wallace on The Office. He was a financial advisor at the time and they wanted him because he looked/sounded like an executive. Totally knocked it out of the park.
Frank Silva as "Bob" on Twin Peaks. He was a set dresser who worked with David Lynch and Lynch liked his look so much they made the character for him. Now he's regarded as one of the better tv villains.
Tom Waits in The Ballad of Buster Scruggs. Sure it wasn't his first time acting but I'd argue he's much more well known as a musical artist and his performance was great.