I second this. When I first saw Crash, and he came on, I commented to my daughter, “Hey, he looks just like that rapper, Ludacris! Where did they find an actor who looks like Ludacris?” She rolled her eyes and set me straight.
Tupac Shakur was amazing in Gridlock’d.
Trumpeteer Alan “Mr. Fabulous” Rubin in the Blues Brothers seemed to have a natural acting talent.
Rob Brown in Finding Forrester . Sean Connery said he wanted an unknown and they essentially picked him up off a playground shooting hoops. Magnificent performance
Paul Simon did a very creditable job as Jonah Levin in One Trick Pony. Of course, he also wrote the screenplay, so he probably had a good head for the character. But Steve Gadd and Richard T. also did good jobs.
The same could be said of almost everyone in the movie The Commitments. The people in the band were basically all picked up in Dublin, and all had ambitions in music rather than acting; I believe only one of them had ever acted before. Yet they all played their parts beautifully.
Mikhail Baryshnikov did a good job as a movie actor after his career in dance. Of course, being a highly trained dancer on stage undoubtedly gave him a leg up (ouch, sorry) as an actor, since he was trained to play a role and express situations and emotions without words.
I totally agree with this one.
However, as has been mentioned several times already, Dr. Haing S. Ngor in The Killing Fields was simply without compare. His own life history was such that the director could go, “Just be yourself,” and get one of the best acting performances of the past half century.
OK, not on quite the same level as some others mentioned, but Aretha Franklin in her small bit The Blues Brothers shows some acting potential. Maybe mostly due to contrast with the other (real) musicians who mostly, well, don’t act so much as recite lines.
Models-turned-actress/actors are usually adequate at best, but Tricia Helfer is outstanding by any measure. TPTB on Galactica can’t say enough good about her.
Before he lost his marbles, Ozzy Osbourne had a small role in a heavy-metal themed movie called Trick or Treat, playing a televangelist. He absolutely nailed the prissy judgemental attitude so many TV preachers had at that time. (Gene Simmons also played a DJ, but that wasn’t nearly as much of a stretch.)
Henry Rollins, who despises cops, has had a couple of small roles in which he plays a cop. It’s hilarious, of course. Shame he’s never been asked to play The Punisher, since he looks exactly like the character, but I suppose he couldn’t carry a whole film.
I have to add Nick Frost.
He was just Simon Pegg’s best mate when Simon wrote Spaced series 1. The character Mike was written around a character that Nick invented to amuse himself and Simon. So Simon cast Nick Frost (who has never acted before, and therefore not an actor) to play Mike. Since Nick has become his own successful actor. Keep an eye out for Hot Fuzz staring Nick and Simon 
Although he acts now - Wolfgang Bodison was orignally a location scout, and I thought he was excellent in his first role in A Few Good Men
Tom Waits does good work. Wnough people have figured that out, and he’s been in enough movies, that by now maybe he qualifies as an actor.
That explains it; I always thought her delivery was terrible.
Dwight Yoakum has turned in enough good performances that he may be considered an actor by now too. And Harry Connick, Jr., too.
Nobody is impressed with Elvis or Arnold. They were moneymakers.
I came in to say that that, but Jim’s Son beat me to it. It was a remarkable performance.
I’ll second Dwight Yoakum. His performance in Sling Blade was very good for someone who was very new to acting.
This brings to mind Alan King, who was a stand-up comedian on the Borscht Belt and a staple on the Ed Sullivan Show. In late middle age he turned to acting and became a very good dramatic actor.
You chose the right word: astonishing. Enough so that she won Best Actress at the Venice Film Festival. She was 4 years old. Wow.
I hate sports, ESPECIALLY American football, but I like many sports movies, and Mac Davis in turned in a surprisingly good and poignant performance in the film North Dallas Forty (I love that movie).
Tim McGraw is doing well as an actor. I liked him a lot in Friday Night Lights (see?) and Flicka.
Tyrese was really good in Baby Boy.
Wasn’t he in some movie with Magnum PI too? It had little robots in it.
Runaway. He was good in it. When I saw it, I was too young to know who Gene Simmons was, but I was scared of him in that movie.
Punk rocker/poet Richard (Dick) Hell, in Smithereens.
Hoagy Carmichael (songwriter, singer, piano man) in **To Have and Have Not ** and **The Best Years of Our Lives ** (where he had a great avuncular chemistry with Harold Russell).