Callis attended the University of York, graduating in 1993 with a BA in English and Related Literature. He was a member of Derwent College, for which he was an enthusiastic rugby player. At university he was also a keen student actor, Director and Writer. He was a key member of the University of York Gilbert and Sullivan society and even appeared in productions put on by friends at Cambridge University, including a production of Harold Pinter's The Dumb Waiter. After graduating from university, Callis trained at LAMDA.