This won't hurt a bit, Mr. Schuester. Actually, it might — a lot. But it's Will's heart, not his teeth, that is most in jeopardy when John Stamos joins Glee in Season 2's second episode as Emma's dentist beau, Dr. Carl Howell.Matthew Morrison's mouth."Carl will also be administering anesthesia to hygienically challenged students Rachel, Artie, Brittany and Santana, inspiring a dreamy sequence that will take them into the world of Britney Spears. "Poor Brittany has cavities in every tooth," says John, who will also appear in the Rocky Horror Picture Show — themed Halloween episode. "I can't tell you what character I'm playing, but I will be singing," he promises. Talk about doing the "Time Warp!"