In ranked online multiplayer mode, either reach rank 10 or unlock the "Let Sleeping Rockstars Lie" achievement. Bring up the cellphone menu, choose "Multiplayer", and scroll down to "Player model". Select it. LB Xbox 360 to access a costume that turns you into a zombie wearing only a Rockstar underwear. When you access this mode, your character will sound like he is moaning and speak in a zombie-like fashion.