Jessica Lange was a fashion model who turned in an abysmal performance in King Kong.
She got better.
Tom Cruise. Need I say more?
The other Wahlberg: Donnie. Way back when I first watched Band of Brothers, I was forced to grudgingly forgive him for New Kids.
Mind you, there’s an asterisk on that forgiveness, since he went on tour with 'em again – 40 year old dads doing tweenybopper choreography and songs is just creepy. But he’s a talented actor.
Going back a bit,David Soul.
He played a sacharrine goody two shoes cop in Starskey and Hutch and sang schmaltzy ballads for prepubescent girls.
Bbut then he appeared as a ruthless,vigilante cop and a generally nasty piece of work in one of the Dirty Harry movies,Magnum Force as I recall and totally went up in my estimation as an actor.
Hugh Laurie. Good British song-and-dance comedian becomes Dr. House.
David Boreanaz in “Buffy” was just a good looking, rather wooden, actor until the episode “I’ve Only Got Eyes for You.” In two seconds, he had to switch from acting the part of a lovesick female teacher (complete with angst and tears), to Angelus, a sociopathic vampire. And I thought, “By gawd, he can really act.”
Yes, yes, YES! I didn’t care much for him as a neutered puppy, but man, when he got going with the evil, he stole the scene as often as Marsters did/does, and really surprised me with his actual acting ability. (Just started Season 3 here.)
Leonardo DiCaprio - Catch Me If You Can
Bruce Willis - Pulp Fiction
Mickey Rourke - Spun
Brad Pitt - 12 Monkeys
Cameron Diaz - Being John Malkovich
Christian Bale - The Mechanic
I know there are others, but those are off the top of my head. It’s always a pleasant surprise to find that people you thought were marginally talented at best can in fact act their asses off. Also, I should note that most of these people have gone on to do amazing work, although some (<cough brad pitt cough> occassionally relapse.
I only know him from Law & Order, but this sounds a lot like what Jerry Orbach’s career was like.
What about Johnny Depp? He started out as a pretty boy lightweight actor on the Fox drama “21 Jump Street” and now I would say, while he is still considered quite pretty, he is far from lightweight
The Machinist maybe??
What I came to say.
I’ll second a whole lot of others as well…Jamie Foxx, Tom Hanks, Brad Pitt, Clooney. Except they 180’d in a good way.
How about the other side - from celeb to knob?
doh! that’ll teach me to monitor multiple threads at once!
:eek:
I only know him as my all-time absolute favorite bad ass on Eastenders. Seeing him mince in that kilt was… um… disconcerting. 
Will Smith. I used to like him, enjoyed his movies and he seemed to be a cool guy. Then he started hanging out with Tom Cruise and I started to have some misgivings about him. Then I heard he is going to remake The Karate Kid and put his kid in the lead. That was when I realized he was just another Hollywood douche.
Paris Hilton.
No, seriously.
There’s a terrible, horrible, no good very bad movie that’s thankfully not in theaters near you called Repo: The Genetic Opera. I won’t go into all the reasons why it’s so terrible right now, but I will say that Paris Fucking Hilton was really damn good in it!
ETA: Oh, and I have a crush on Victor Garber like you wouldn’t believe. I cannot for the life of me figure out why he makes me all swoony.
Not to argue with your opinions, but you might want to get the time line straight to figure out which direction that 180 turned. 
**Colin Farrel. **I saw him in SWAT, Phone booth, and Miami Vice. I then vowed to never watch another Colin Farrel movie ever again.
Then I heard good things about In Bruges and decided to give him one more chance. He was awesome. I totally did not expect that kind of performance from him.
Heath Ledger’s Joker was also impressive.
Heh- David Soul is going back a bit and I mentioned an actress who died in 1937? Now I feel positively ancient- thanks so much 