It's a movie! Yeah, we're gonna make a movie!

Coming in very late--

I have picks for only a few of the roles, because I haven't yet reread R&R in anticipation of picking up the new book. However:

Toby:
Molly Quinn from Castle, necessarily aged a few years.

If one is trying to visually approximate the book covers, there are two instantly obvious contenders -- Eliza Dushku and Kristen Kreuk (Smallville), but Kreuk doesn't have nearly the acting chops, and Dushku seems to me to bleed too much of herself into her roles. The thing about Molly Quinn is that while she has criminally low screen time on her current series, there are indicators every so often that she has considerably more range than Castle allows her to display. And she does so very well as the no-nonsense Good Girl (Alexis) that I think she'd nail the no-nonsense Damaged Girl that is Toby just as solidly.

As a possible runner-up (and bearing in mind that I've not seen True Blood at all, and so am extraplating far more from her X-Men appearances), Anna Paquin could work well for me.

Devin:
Daniel Radcliffe, probably also aged a few years (but not too much).

As admnaismith points out, Devin's very tricky; his character arc requires someone who can play both as a mentor figure and as an equal to Toby, and someone who has both charm and darkness. Which doesn't necessarily make you think of Daniel Radcliffe -- until you remember his stage turn in Equus. Of the names above that I'm familiar with, most seem to be best known for playing either villains or difficult personalities -- but I think an R&R movie needs a Devin that you don't expect to develop the way his character actually does, and I think Radcliffe as Devin would be both a very strong acting choice and one that would not unduly foreshadow the movie's outcome.

Tybalt, King of Cats:
Neil Patrick Harris.

Here again, admnaismith makes a lot of sense; Tybalt needs an actor with chameleon-like qualities and a degree of zaniness. His suggestion of Jim Carrey and sheistheweather's of Johnny Depp are both well thought of (Depp especially), but both are way too old to play opposite my other choices. NPH is a bit younger, can play younger than he actually is...and, I think, has the acting chops to unveil further layers of complexity in Tybalt's character as the books develop. And I think he'd have excellent screen chemistry with the hypothetically grown-up Molly Quinn.

Danny the Bridge Troll:
Keith David, best known for his voice work as Goliath in Gargoyles.

the Luidaeg:
Susan Sullivan (Martha Rodgers in Castle -- both because it would be highly amusing with Molly Quinn playing Toby, and because I think she'd do an excellent job in the part.