[Alexander]

Xan must be played by Alan Rickman.

Gorion must be played by Anthony Hopkins.

Elminster must be played by either Ian McKellan, Sean Connery or Richard Harris.

Garrick would probably be some annoying twentysomething male heartthrob - like, say, Leonardo DiCaprio. Hopefully he'd get killed.

And Jon Irenicus? Jeremy Irons. He would be perfect in the role - heck, they even have the same initials. I would say David Warner, if they could do it Darth Vader style, with one person as the voice and another as the body (although I kinda get the feeling that Sarevok would be like that, too).