This was the cover of Guards! Guards! I first saw, so this is in part how I always picture the core of the Watch:
Left to right, that's Nobby, Carrot, Vimes, and Fred. It's probably also why I always thought of Alun Armstrong. I knew him originally from Krull, back when he was thinner and had hair:
I kind of picture Sam looking more or less like this, especially when he's drunk, until he marries Lady Sibyl. Then he cleans up a bit, but only a bit, and that grudgingly to please Sibyl.
I've had the hardest time coming up with someone for Carrot, possibly you need someone who's so damned earnest all the time, but who also has more going on than he seems to. Don't know Hall well enough -- never really watched Dexter or Six Feet Under -- but interesting. I actually love the idea of Adam Baldwin -- he does the big boy scout thing well in Independence Day, and he does the (more than) slightly devious thing well as Jayne in Firefly. He's certainly got the physicality, too. But somehow, I always picture someone very, VERY British, almost parodically so, like young Hugh Laurie in Blackadder.
Oh, and I'm with you 100000% about no David Jason.