Commander Shepard:
Now you can go pretty much anyway with this one because he is the hero of the game and you get to create him at the beginning but i think that they should go with Matthew Fox.
Kaidan Alenko:
He's a biotic which means he can use element zero to manipulate mass and move objects just through electrical implants in his brain. He's good friend of Shepard's.Leo Dicaprio would be perfect for this role.
Ashley Williams:
She's a soldier that joins you like halfway through your first mission. She's one of those tough girls and a very good soldier (she seems to have a chip on her shoulder). She also is one of the love interests for Shepard in the game. Emily Deschanel looks the part and would be well suited for this role in my opinion.
Garrus:
He is what they call a Turian in the game they are the first aliens that the humans made contact with. He's very good at what he does but thinks that he should be able to get the job done however he can even if innocents are involved(he proposes some moral choices to Shepard in the game) Zachary Quinto is a really great actor and I think he would be good for Garrus.
Urdnot Wrex:
Wrex is a krogan which is a species that have driven themselves out of their homeworld due to nuclear warfare. Most of them just want to fight for money pretty much everyone of them you see is a mercenary. Kevin Grevioux is perfect for this role.
Tali Zorrah Nar Rayya:
Her species the Quarians created an artificial race of machines that turned on them when they started asking questions about how they became what they are and what not. The Quarians were forced to leave their planet due to the conflict and now they travel across the galaxy looking for a suitable planet to start over. Due to the compressed controlled environments of their ships the Quarians have to wear pressure suit at all times outside of the Flotilla(what their group of ships is called) because they have a very weak immune system. Ellen Page is a good choice she has the size and build for the character.
Dr. Liara T'soni:
Her species are known as the Asari. Her people are all female but can mate with any species or gender. She is one of the love interests for Shepard. She is a powerful biotic and also the daughter of one of the villians. You find her on a planet while she is researching artifacts of a long gone race known as the protheans. I think that Rachel Weisz could pull this role off with ease. She's a great actress and has the look if you painted her and what not lol.
Well there you have it let me know what you think in the comments.
(I got my pictures from google.com)