People I for sure have set in my head when I read the books:
Marcone: Eric Roberts
http://www.imdb.com/name/nm0000616/
Sanya: Adewale Akinnuoye-Agbaje (Mr Eko from Lost)
http://www.imdb.com/name/nm0015382/
Father Forthill: Henry Gibson
http://www.imdb.com/name/nm0002099/
Morgan: Billy Connolly
http://www.imdb.com/name/nm0175262/
The rest tend to be people I constructed from the descriptions in the books they dont really match actors off the top of my head. But the above were dead on for me no questions they're who I see when I read. Also if they do a movie James Marsters has to voice Bob no questions.
I'd say that Adewale Akinnuoye-Agbaje and Henry Gibson are both on point with how I picture them, except that Adewale might be too old; I picture Sanya as around Harry's age.
Roberts smiles too much for Marcone, and he's only sinister when he smiles. For Marcone, you need a sinister look with a straight face.
I picture Adrian Pasdar as Marcone. He's 47 now, so a touch of grey at the temples will be about right.
And I'm a Billy Connolley fan, but that's not how I see Morgan. I honestly can't say who I picture Morgan as, because he's always just been this big, older wrestler-type. Someone that has the height of Harry, but a lot of muscle. And a bad attitude. But since I can't think of anyone, oh well. In the meantime, here's a picture of Mark Calaway, a.k.a. The Undertaker...
And on a random tangent, Colin Salmon is how I picture Warden Chandler, a.k.a. 'Steed'. If you've seen the James Bond film
The World is Not Enough, he played Charles Robinson, the chief of MI6. He had that stuffy British ministry vibe combined with a bit of humor, which is perfect for Steed.