So here are some things I want to say initially, concerning the movie:
1) I'm not concerned with casting an exact default Shepard look-alike. That's stupid, and uncreative. This is a fancast, remember?
2) See Rule 1. Rule applies to other characters.
Simon Pegg as John Shepard
I think Pegg is a good choice for the role, since Shepard's physical properties save for his voice in the game is open to the player's discretion.
Pegg is a good actor, and his acting style would be refreshing to see, and he could really pull a fantastic performance if he were serious.
Joaquin Phoenix as Kaiden Alenko
I think Phoenix is an obvious choice for Alenko, seeing as they have remarkably similar facial qualities. But Phoenix is a flexible actor, and he would fit Alenko's nonchalant attitude.
Seth Green as Joker
.....duh.
Jackie Earle Haley as Garrus Vakarian
I choose Rorschach because he has the right voice of Garrus. I do not list him as "the voice of" because I think he could be makeup-ed to look like Garrus, like Worf from Star Trek TNG.
Kristin Kreuk as Ashley Williams
For one, Kristin Kreuk would be a great addition to the cast because she is young, and recently famed for her role as Lana Lang in Smallville (hush Haters, this is not a Smallville article *chokes random Hater).
Kristin is also a versatile actor, not to mention easy on the eyes :)
Milla Jovovich as Liara T'Soni
I was unsure about this rule, since the actual character has rather a fat face, and she does not. But looks arent everything. She already played a blue character in X-Men, and she did fine.
Dolph Lundgren as the voice of Urdnot Wrex
Again, a hard role to cast because Urdnot is a Krogan, and not a human. He deviates so much from the human form, a CGI character might be neccessary. Dolph has the kind of voice that could possibly fit the character. I'm open to criticism on this one.
Zoe Saldana as Tali'Zorah Nar Rayya
Hmm....we don't know what Tali ACTUALLY looks like. So why not cast the hot chick from Avatar? Tali has sort of a middle-eastern voice too, Zoe did a good job with her voice on Avatar.
Closing Remarks:
I don't believe fully in CGI. I think the movie would be sweet without some of the main characters being computer generated. A fully functioning suit would look phat on screen, for lack of better words.