David Tennant: I remember when they were filming but I wasn’t on camera, but filming my half of that scene… James Hawes, the director, he came up to me and said, “Come just make them laugh. Be off camera and make them laugh.” 
Julie Gardner: Did you? What did you do?
David Tennant: I got Steve, my make-up artist, to paint a big mustache on my face. It seemed to work.

(School Reunion episode commentary)