Willow: Oh, yeah, I’m good at medical stuff since Xander and I used to play doctor all the time.
Xander: No, she’s being literal. She used to have all these medical volumes, uh, and diagnosed me with stuff. I didn’t have the heart to tell her she was playing it wrong.
Willow: Wrong? Why? (to Buffy) How did *you* play doctor?
Buffy: (evasively) I never have.
Buffy the Vampire Slayer – Season 2: Episode 18 – “Killed by Death”