Gambon plays Hogwarts headmaster Albus Dumbledore, who was recently outed as gay by Potter author JK Rowling.
Asked by Parkinson how Michael had reacted to the news, Radcliffe replied: "He's been camping it up around the set like nobody's business!
"Not in the film, though we did actually film a scene the other day, and I would like emphasise at this point that the script was written before this revelation and I don't think Steve Kloves had any prior knowledge, but there was a scene the other day when Michael had to come in and tell another character how much he loved knitting patterns! Which he just adored saying that."
Parkinson airs at 10.45pm tonight on ITV1.