Unlock Zombie costume
In ranked online multi-player mode, either reach rank 2 or kill a Rockstar employee. Bring up the cellphone menu, select "Multiplayer", then scroll down to "Player model" and choose it. Press L1 to access a costume that turns you into a zombie wearing just Rockstar underwear. When you access this mode, your character will sound like he is moaning and wont be able to speak.