When movie audiences were first introduced to Professor Charles Xavier in 2000's X-Men he was bald, just as he is in both the comics and cartoons. Portrayed by Patrick Stewart - an actor famous for his striking baldness, having lost his hair as a teenager - the leader of the X-Men was a bald man in a wheelchair with incredible psychic abilities.
But when it came time to explore Xavier's origin in X-Men: First Class, the younger Charles was played by the decidedly not bald James McAvoy. He also wasn't bound to a wheelchair, though the film did eventually provide an explanation for how he became paralyzed. First Class skirted the issue of Charles' impending baldness, as did its sequel, X-Men: Days of Future Past. In next year's X-Men: Apocalypse, however, McAvoy is going bald and the film will finally explain how it happens.
On a recent episode of The Graham Norton Show that included McAvoy and his Victor Frankenstein co-star, Daniel Radcliffe, the host asked the actors about their recent stints being bald. Radcliffe had just shaved his head for a role as a CIA agent who infiltrates a group of Nazi skinheads in Imperium, and McAvoy had finally adopted the classic Professor X look for Apocalypse. Norton then asked McAvoy if Xavier goes bald for a good reason or "is it really dull?"
"He ends up going through something so horrible and painful that he literally half pulls his hair out half… it falls out," McAvoy explained, though added, "maybe or maybe not," in an attempt to cover his tracks. Norton followed up by questioning McAvoy if he should have said anything, to which he replied: "I just sh-t myself because I know, Fox Studios who own me might be angry with me for sharing that."
As for what would be "so horrible and painful" that he'd lose all his hair, well, there's every chance it relates to the film's immortal villain - Apocalypse. The film's synopsis bills him as the 'first and most powerful mutant,' so clearly the X-Men are in for a quite a challenge when they face off against him and his four horsemen.
In the comics, Xavier loses his hair at an early age as side effect of his mutant powers, but with McAvoy playing a roughly 40-year-old Xavier in Apocalypse, the event that makes him bald must be a traumatic one. Perhaps the film sees him stretching his abilities to their limit? Or maybe he has his psychic powers put the test, but they fail, resulting in the hair loss. Almost anything is possible at the moment, but there may be a clue found in the upcoming X-Men Apocalypse teaser, coming to theatres with Star Wars: The Force Awakens.
Deadpool opens in theaters February 12, 2016; X-Men: Apocalypse on May 27, 2016; Gambit sometime in 2017; Wolverine 3 on March 3, 2017; and some as-yet unspecified X-Men film on July 13, 2018. The New Mutants is also in development.
Source: The Graham Norton Show (via Bleeding Cool)