Step one is to become a zombie. Step two, rampage around a lowbie zone. Step three, scream "Braiiinnnns!" for everyone to hear. Step four, wait for an immersive roleplayer named "Pwnsurface" to kill you, in accordance to his Retribution Paladin's most tightly held beliefs.
Okay, there's a little more to roleplaying the event than that. It's tempting to launch yourself at the suspicious crates and transform into a brain-hungry beast. But to borrow a quote from a roleplayer on Shadow Council, if you're going to do that, chances are that what you're really doing is "taking an OOC day."
Most characters in the Alliance and Horde aren't yearning to find themselves transformed into an instinct-driven cranium muncher. Zombies are not only evil, they're members of the Scourge, out to destroy all life as we know it. They also have fairly bad personal hygiene. (Okay, so maybe they're not that much different from the Forsaken.)
But to us players, Zombies are cool. You get to do stuff, and attack people, and explode, and convert people into zombies, and it rocks! And while your character might find itself outta bubblegum, and decide to start kicking butt instead, you-the-player might feel bad for ruining someone else's good time.
Still, there is roleplay to be had here!