He's been acting for a while, mostly in pretty amazing comedies (check out This Is Spinal Tap for one of his earliest and most famous films). This dramatic turn is pretty incredible though.
I agree about Gilligan's casting choices, everyone just totally owns their characters. A lot of times when I'm watching movies with recognizables actors, I feel like it's hard to forget that it's Tom Cruise or Matt Damon or whatever. But even with all the previous work I've seen of Bob Oedenkirk or Michael McKeane, they completely disappear into their roles on this show. That's pretty rare.