Though a long time fan but new to the forum, here are my two cents on casting for a movie;

Harry - Nathan Fillion
Thomas - Timothy Olyphant
Butters - Zack Gallaphinacas
Merlin - Jeff Bridges
Murphy - Sonya Walger
Michael - Richard Dean Anderson, or James Marsters
Sonja - Christopher Judge
Luchio (Old) -
Luchio (Young) -Ashley Green
Molly - Dakota Fanning,
Mouse - Wooly Mammoth (dyed gray)
Billy - Zachary Levi
Ramirez - Taylor Lautner (he could pull it off)
Harry's Land Lord - Betty White


Thats all i could pop off the top of my head, though final thought and I know many have had it, I think James Marsters should play Harry, or at least someone in the Series maybe Michael