Action, man!

Sometimes an actor is so memorable in a role that he or she is forever associated in your mind with that character. Here is a list of the actors I cannot distinguish from their roles.
Pacey (aka Joshua Jackson)
Before he was Pacey he was Charlie from the Mighty Ducks, so I've been a fan for a long time. Now he's doing heaps of cool things on Fringe, probably my favourite show in the world at the moment. But after the Ducks and before Fringe, he was Pacey. Dawson's best friend and love rival. The teacher-romancing, restaurant-running, all-sailing, all-sarcastic Pacey. And he got the girl. Sigh.
Stringer Bell (aka Idris Elba) He's been showing up in recent US Office episodes and been very funny, but it took me a moment to remember his character's name. I just referred to him as Stringer in my head whenever he shows up. He was in RocknRolla (still String to me!) and in some crappy horror movies (Stringer always was a badass). I don't know if he will ever not be Stringer to me, unless he inhabits another role so completely.
Ian McShane (aka Al Swearengen and King Silas)
This one is different. Because in both roles he appears to be playing 'Ian McShane does drama', which is awesome to watch. If you put Al Swearengen in the capital at Shiloh, told him to lay off the swearing and the running of whores, he would pretty much be King Silas. Deadwood was great mainly due to him (and sheer white hot intensity of Timothy Olyphant's sherriff), and Kings is great because of him too. So when I watch those shows, I don't see the character. I see 'Ian McShane does a great job, doesn't he?'.
Boomer/Sharon Valeri (aka Grace Park)
I'm still only halfway through Battlestar Galactica (my sister and I have taken a break from it for a while) but whatever happens, I don't care. My love for Boomer/Sharon Valeri is strong. Ohhh the tension. Ohhhh the drama. Ohhhh the love. Damnit Sharon! Damnit Boomer! I need to watch some Battlestar.
GOB (aka Will Arnett)
He's been doing a series of bad movies since Arrested Development, but occasionally he does something great. Either way, when he shows up, I say 'Heh, GOB.' That voice. He did a great few cameos on 30 Rock (as Jack's gay rival within GE), but even there he was basically playing GOB with a penchant for dong. It must be tough for him, having found a role that suits him so well that everyone writes parts for him where he plays that role. It's nice he's getting work, but I wonder if he'll get tired of it. I'm not!
Dr Cox (I think he's John C McGinley?)
I guess he's been in heaps of stuff, but everyone knows him as Dr Cox. He'll always be Dr Cox.
Daniel Jackson (aka Daniel Jackson)
This one is also different, because Daniel Jackson (from Stargate) is clearly playing himself. He takes time off from deciphering stuff for the US Government to play a somewhat fictionalised version of himself on TV. It's pretty cool. I guess they figured he's so invaluable they'd let him do what he wants. But you can see why I find it hard to separate the two of them when he's actually the same person.
Whitney! (aka Nicole Bilderback)
She's the Asian cheerleader in Bring It On ("W-W-W-Whitney!), but was also the cool Asian girl in Clueless, her first credit. Anyway, she shows up on TV all the time. Five episodes of Dawson's Creek, appeared on Cold Case, Without A Trace, Boston Legal, House, Heroes and a bunch of others. And whenever I see her, she's Whitney!
Karen (aka Megan Mullally) I have not seen much Will & Grace (thankfully), but I do know who Karen is. And she can be funny. And whenever I see her anywhere else? She is Karen. Not Megan.

Anyone else?