The main characters from what I understand will be: Superman, Wonder Woman, The Martian Manhunter, Green Lantern, Batman, Flash, and Aquaman.
These are the rumoured people they are thinking of to play parts:
Jake and Maggie Gyllenhaal: Wonder Twins (as a cameo)
Wonder Woman: Last we heard Jessica Biel was in talks for this one. Lord helps us if she is.
Batman: I have heard talks of Ryan Gosling
Superman: There are rumours that Tom Welling will play Superman. Maybe if he puts on the suit somewhere I'll think of him as superman then.
Flash: Ryan Reynolds. This is the first person that I am actually happy with on their possible casting.
NONE of the above have actually been confirmed as far as I know though.
Also rumours were flying this was going to be shot in motion capture CGI... but it has been announced it will only be partly shot like this (as most movies are).