The apartment buildings landlord, Mr Henderson is unchanged when Sam first finds him, but the creatures that are attacking him eventually cause him to mutate.  He is very suspicious of Sam, and when he begins to change, he becomes obsessed with him paying his rent.  As time goes on, he becomes more unhinged, before finally attacking Sam as a giant monster.  He should have a fairly stereotypical elderly voice to start. But it should warp and distort as he changes.