Kevin Wendell Crumb - James Mcavoy
James Mcavoy was basically playing multiple characters at once. Kevin Wendell Crumb is a man who has multiple personalities that he can't control. The most threatening character is The Beast, who has superhuman strength and takes this film into the supernatural realm.
Peter Pettigrew - Timothy Spall
Peter Pettigrew is the human reincarnation of Ron Weasley's pet rat. He's one of the earliest villains in the Harry Potter franchise. Spall's costume was a combination of special FX and makeup, minus his transformation scene. That was all CGI.