I knew he played Doctor Who and as Barty Crouch Jr, but I didn't know about this. I'm currently rereading Harry Potter. I'm in the middle of Prisoner of Azkaban, but I'll be sure to check the chapter when I get to Goblet of Fire.
His name is The Doctor, not 'Doctor Who'. It's called Doctor Who because every one asks that question when he tells them his name. "I'm the Doctor by the way." "Doctor who?" "Just the Doctor."
Everyone just calls him "Doctaaahh!". Well, screams.