brad garrett
Duurr i tink Steven Downes is the guy that played terminater. lol
Nah Steve is the dude that played Master Chief
I think David Scully should voice Johnson at least, and perhaps act him as well, depeneding on if they can make him look the part and if he is a good actor. As for Cortana, I believe Jen Taylor should voice her, but Lexa Doig would play the actual acting (because they look so much alike. See: http://forums.halowars.com/thread/16142.aspx).