As for Jacob, I think Michael Copon would make an excellent older Jacob.
I mean, I loved Taylor Lautner but I really think he would have made a better Seth, myself. His look is way too young, way too youthful to play some deeply conflicted, angry, secretive werewolf type person. Not too mention that Michael Copon could really be Taylor Lautner's older brother so the transition wouldn't really be all that difficult to swallow.
Who knows? Maybe Taylor will surprise us. He really didn't have much of a shot in Twilight.
As for the Voltari...
I think that Ben Barnes and Cillian Murphy would make absolutely AWESOME Voltari. I don't even care which ones they play so long as they're in.
I don't know, they seem to have perfected that balance between beautiful, dangerous and perfectly caustic and bored. Either of them or both of them (a girl can dream...) would be perfect.
I think actors like Ian McKellen and Christopher Lee though--for as much as I can appreciate their talent would be all wrong for the parts.
Both of them are way too old. And really, if I were an up-and-coming actor, I couldn't imagine doing any sort of scene with either of them. Vampires are supposed to be young and seductive and beautiful and while McKellen and Lee both just OOZE power with their presence. I don't see anyone over the age of...about 31 playing a vampire in this saga.
It would be hard to resist a movie with a Christopher Walkin vampire though...
As for Leah, the closest I can come to visualizing a Leah is Michelle Rodrigues. Yeah, I know, she's not exactly your classic beauty but she does have that strange pretty-yet-can-and-will-rip-your-head-off-in-a-second presense. Though, any of the previously mentioned actresses would be pretty good too.
As for Jane and Alec. I have no idea who I would chose, Kristen Chenowyth has that perfect childlike presence (and that amazing voice) and she is short as heck. Somehow though, she just seems so.....CUTE. I can't see her as a bitter, angry, ready-to-kill vampire. Has anyone seen her in a fairly serious role? It would be interesting to see her pull it off.
Alec, well, gosh. The only person I could see in that role isn't an actor. He's a barista in Long Beach. And I don't see him getting into films any time soon...