Ron Swanson / Nick Offerman. He does nothing for me as Ron, but when he's being himself (or in other roles), uh, yes.

I'm with you there, except now I see the actor playing a role so I just like him anyway.

More along the lines you were thinking of, and from the same show: Andy = yes. Chris Pratt = not so much.

Chris Pratt has played a lot of assholes, but now I just see the actor, again.
Another Mad Men bit, Connor yes, Pete no, Vincent no. And no Dick Whitman, just Don. Dick moment melt away all attraction.

Speaking of, well, "being himself", I can't stand Tom Cruise playing anything but an asshole (Color of Money, Rainman) or Leonardo DiCaprio as anything but a kid or mentally-deficient or preferably both (What's Eating Gilbert Grape). I can't even fathom DiCaprio as any kind of hero, especially in the Gangs of New York fiasco that somehow netted Daniel Day Lewis (who, like Guy Pearce, can take any part whatsoever, male or female, although hopefully they're both above playing James Bond). I'll admit I didn't hate DiCaprio in a couple films, though.

Sorry if I answered as if you'd asked, "Who would you like to see in a red shirt on Star Trek?"

I never thought DiCaprio could act before the Departed.