I'm not going to waste your time with a lot of bloated commentary. Let's get to the pics... 
Invincible/Mark Grayson - Douglas Smith 
 
This role calls for an actor with range. Humor, intensity, anger, fear, heroism; Invincible needs to exude all of these things and more. I've seen Douglas Smith on Big Love and this kid has serious game. He could act circles around just about any young actor out there. 
Omni Man - Jeffrey Dean Morgan 
 
Atom Eve - Evan Rachel Wood 
 
The Immortal - Liam Neeson 
 
Props to Ryden who also liked Neeson for this role in a previous fancast. 
Rex Splode - Dave Franco 
 
William - Jesse Eisenberg 
 
Robot - Doug Jones 
 
Dupli-Kate - Jamie Chung 
 
Monster Girl - Isabelle Fuhrman 
 
Allen The Alien – Patrick Warburton
Cecil Stedman - Craig T. Nelson
Angstrom Levy - Don Cheadle
Amber - Amber Heard
Art - John Mahoney
Mark's Mom - Minnie Driver
And, just for fun...
Science Dog - Bruce Campbell!
And finally, this cast would not be complete without Mark's Principal Mr. Winslow. Anyone who has ever done the Urkel dance should appreciate the cameo appearance of this superb actor in the early comics who Mark's school happens to be named after AND who starred in the television series from which the first volume of the Invincible trades gets its name...
Principal Winslow - Reginald Veljohnson!!!!
BOOM! Done.