I have never liked Keanu Reeves in anything (Other then Bill and Ted’s Excellent Adventure & Bogus Journey) until I saw THe Replacements a few weeks ago. He actually ACTED in that movie GASP!
Orson Bean. He played the old man in Being John Malkovich* and was terrific. I always thought of him in terms of his Match Game/To Tell the Truth participation so his performance in this movie was a wonderful surprise.
Orson Bean also had a great guest performance on “Will & Grace” this year as an old college professor of Will and Grace (who had been a wonderfully positive influence on them), who they went to visit and saw what the two of them were going to be like as they witnessed the professor squabbling constatntly with his best friend.
Brad Pitt permanently erased his pretty boy image in “Kalifornia”. He gave an absolutely awesome, surprising performance.
If you haven’t seen “Kalifornia” I highly recommend it. Juliette Lewis, Brad Pitt, David Duchovny all put in great performances. But Brad Pitt really is outstanding. He’s barely recognizable, and exudes pure menace all the way through the movie.
Reese Witherspoon blew my socks off in Election. Yes, she was good before that (in Freeway and as The Naked Girl in Twilight), but I wasn’t prepared for her work as the uptight highschooler.
Cameron Diaz in Being John Malkovich. Yes, a pretty face. Yes, very good comic timing from the get-go (c.f. The Mask). But who knew she could deliver such a bundle of de-glamorized neuroses?
This goes back a few years, but perhaps the ultimate “I didn’t know he could do that” breakthrough performance was from Kurt Russell in Escape from New York. Before 1981, remember, he had been trying to shake off the mantle of Disney Kid (e.g. The Computer Wore Tennis Shoes) for a full decade.
Oh, and if you want a surprising Robin Williams performance, check out Dead Again.
On the disappointment side:
Neither Salma Hayek nor Catherine Zeta-Jones has lived up to their initial levels of promise. The former burned up the screen in Desperado, and the latter was a firebrand in The Mask of Zorro. Both have been amateurish and lame in almost everything since – see Time Code and Entrapment, respectively.
There’s sort of a corollary question, here, that might warrant its own thread, regarding actors who used to be good but who haven’t really challenged themselves lately or whose talents have gone untapped. Mel Gibson is a good example; think back to Gallipoli and even the first Lethal Weapon movie. Compared to those, Braveheart is coasting. And then there’s Rutger Hauer…
There are a lot of great actors who simply can’t read a movie script and tell that it sucks, or who are getting bad advice from agents, or whatever. But they consistently show up in 3rd rate movies. Michael Caine comes immediately to mind. Rutger Hauer had an amazing start in BladeRunner and The Hitchhiker, but after that appeared in an endless string of lousy movies.
I thought William H. Macy was a milk-dud until I saw him in ‘Fargo’. And Brad Pitt until ‘12 Monkeys’.
Ben Affleck, usually lame, was great in ‘Shakespeare in Love’. And Rob Lowe in ‘West Wing’ is very suprising.
To be honest, I had forgotten a lot of Brad Pitt’s earlier performances before I posted the OP.
My apologies to Brad if he posts here. 
Courtney Love really suprised me when I saw her in The People vs. Larry Flint. Excellent acting job, IMO.
Ms Love? Acting? I thought she was being natural in that coughmoviecoughhackwheeze…
I’m going to gargle with acid now…
dhanson: Rutger Hauer got his amazing start well before that, with a few European films, notably Soldier of Orange.
Here’s another one that goes back a bit: Remember when Sean Penn was a young prettyboy, from Fast Times at Ridgemont High and Taps? He was regarded as a good actor, but nothing to write home about. He gave some clues in Bad Boys, and then came The Falcon and the Snowman. Kaboom! Suddenly he was a Real Actor.
A lot of people were surprised by Michael Douglas’s lumpy and good-humored turn in Wonder Boys, after his string of Gucci-clad slimeballs.
Oh, hey, how about Melanie Griffith in Something Wild? Still a career highlight, as far as I’m concerned.
And don’t forget Al Pacino’s clowny-clown performance as the bad guy in Dick Tracy…
I totally agree. I HATE Kevin Costner as far as one can hate a person for being a miserable actor, but I was completely shocked by the quality of this film. I loved No Way Out but gagged over everything he did since then until Perfect World.