I'd turn around and laugh and be like 'Dude, that's an awesome costume. You can stop trying to scare me now. Hey man, seriously, it's a funny joke but stop touching me. What the, STOP BITING ME OOOOAAAAAAAHHHHHHHH MY FACE IS BLEEDING AAAAHHH MY NECK IS SNAPPED AAAAAAHHHH IM DEAD!'
Being cynical like I am, I wouldn't be able to comprehend that it's real, and just think it's some idiot in a costume. 99 out of 100 times I'm right. But that one time...well, at least it'll be the last time I'm wrong about something!