I have had long discussions about this with many friends of mine, and cast a great many characters in my head over years of reading. Wish I'd gotten in on this thread a little earlier, but what the heck, here's my list:
Harry I've pictured as Nathan Fillion for several years, so I'm stoked that he leads the poll on this thread. I've seen him be deadly serious and total wiseass, so he for sure has the range, and he's tall (not close to seven feet, but certainly over six feet, I've met him) and I think he looks like Harry.
Now, I have heard Jim say Erica Cerra is Susan, and I'm not saying that would be a bad call, but I always picture Gina Torres, myself.
I like Renee O'Connor as Murphy. I kind of picture Melissa Rauch (if not her voice) as Murphy most of the time, because she is blond, and cute, and literally five feet tall. For fight scenes, though, Renee is doubtless the better choice.
I have an interesting choice for Marcone; Sean Maher. I keep thinking it doesn't work because he's rather young, but then when I read the books I keep picturing it. He has a certain skill for self-contained intensity that just resonates with my idea of Marcone.
I've pictured Michael as Alan Tudyk almost since he appeared, mostly because he's the only actor I can see delivering those lines without irritating me, but I also think he has the right structure for it, especially when he's been working out. (I have a very clear picture of Charity in my head, but I can't think of anyone who could play her, apart from maybe Carrie Fisher back in her heyday.)
Lea has always appeared as Christina Hendricks to me.
Molly is tricky, because she grows up over the course of the books. Grown Molly I kind of picture as (younger) Rose McGowan, though. I don't know who the heck I would cast if it were going to start filming today.
I have a lot of trouble casting Ebenezer. I can picture him just fine, but I can't really name anyone who looks like what I see in my head. I think the closest might be Rip Torn.
Just because he has proven himself so good at playing evil, I tend to picture David Boreanaz as Nicodemus.
Butters is totally David Krumholtz. He's even got the hair for it.
The Mab of my imagination kind of fluctuates between Juliet Landau and Oliva Wilde.
Many people seem to agree with me on Thomas. Always always pictured him as James Marsters. Difficult not to, really. For Justine I think Kat Dennings comes closest to what I see in my head. (Oh, and I totally agree on Cillian Murphy as Madrigal, that's a great call)
Enver Gjokaj as Ramirez.
D.B. Woodside as Sanja, though I kind of love the idea of Old Spice guy, as well.
I think Christina Ricci would come closest to my picture of Lasciel, though I suppose her appearance changes from time to time.
I think I see Elaine as Claire Danes. Or Amy Acker. I know, they're quite different.
Gard I see as Katee Sackhoff.
I've seen Angelina Jolie mentioned for Lara. I can't really disagree with that.
I only skimmed the thread, but I haven't seen many names put forward for Luccio. I would submit Patty Duke for pre-swap, and Shirley Yeung for post-swap (though, I'm not sure if she can speak English or not, but she's just exactly what I picture, and this is all fantasy casting anyway.)
I'll agree with Clancy Brown as Morgan.
I think I'd like to see Hugh Laurie as the Merlin. He does irritation so well.
I'm sure I'm missing a few, but it's getting late and I can't think of anyone else just now.
P.S.
After watching Leverage, I think Christian Kane would make an excellent Kincaid. I know people have already suggested him, based on Angel, but I've never seen it, so I had nothing to base that on.
Yes! I could totally see that. I think of Kincaid as a little taller, but I could still totally see that.
(If you didn't notice the pattern in my own casting picks, I kind of have a fondness for almost anyone who was on pretty much any Whedon show. What can I say? They tend to be cast fairly well in nearly all cases.)