My (constantly changing) list, in order of cast choices I feel most strongly about to least (and I'm trying to imagine a cast that could actually be afforded, so being less well-known is actually a plus):
Lasciel / Lash -
Tricia HelferAnyone who's seen Battlestar Galactica knows Tricia Helfer as the amazing actress who played the cylon Six, whose many varied versions ranged from devious seductress to timid victim, from cruel and imposing to frightened but hopeful. After rereading one of the novels with Lash for the first time after watching BSG, I imagined every line spoken by Tricia Helfer, and it sounded perfect. I know without a doubt that she could pull off the deadly temptation of a fallen angel, whose temptation may not always seem so... unkind.
Johnny Marcone - Jim Caviezel
http://www.imdb.com/name/nm0001029/Cold, hard, but handsome; driven to criminal extremes by a tragic past; ruthless, unrestrained, and dangerous to betray. The Count of Monte Cristo, or the gentleman Johnny Marcone. A criminal scumbag with a steely smile that will make Harry look awkward and juvenile in comparison, because, honestly, there's few men cooler or cuter than Jim Caviezel.
Anastasia Luccio (post swap) - Vera Farmiga
http://www.imdb.com/name/nm0267812/?ref_=tt_cl_t7One of my favorite actresses for her performances in Departed and Up in the Air, I can easily imagine her playing the long-in-years Luccio in a more youthful, exuberant body.
Michael Carpenter - Leonard Roberts
http://www.imdb.com/name/nm0731346/I've only ever seen him in Heroes and Drumline, but for some reason I always imagined he'd be a good Michael, though I'm not sure how many readers were like me and imagined the Carpenter family as African-American (Molly's blonde hair was from dye).
Nicodemus - Christian Bale
http://www.imdb.com/name/nm0000288/The biggest name actor on the list, the most expensive, the most exorbitant... and completely worth it, when you're looking at a character who might be the most evil.
Bob - Peter Dinklage
http://www.imdb.com/name/nm0227759/Did I say the biggest name actor on the list was Christian Bale? Scratch that. PETER DINKLAGE!
Karen Murphy - Julia Stiles
http://www.imdb.com/name/nm0005466/Billy or Butters or, I don't know, somebody - Ben Whishaw
http://www.imdb.com/name/nm0924210/