AT the start of this reminiscence of his life and career, Ian McKellen dwells upon the strange business of trying to find the real person behind the celebrity image.

On talk shows, for example, “You’re announced, your public persona is briefly described and on you walk... It’s very difficult to be yourself, so I treat it as a bit of acting – what side of Ian McKellen am I going to present?”

It’s a typically honest insight from the actor and an astute way for Joe Stephenson to open his fascinating and beguiling portrait of him. For beyond the general observation is a truth specific to McKellen, who for much of his life as a gay man in an intolerant society struggled to be himself, lacking confidence everywhere than on the stage, editing what the world knew of him.

As he turns 79, that scenario is altogether different. McKellen didn’t become a household name until he was near 60, with his blockbuster appearances as Magneto in X-Men and Gandalf in The Lord of the Rings trilogy. But today he’s a national treasure, known both for his performances and his fierce defence of gay rights, whose personality positively bursts through the controlling artificiality of the showbiz world.

And it’s the roundedness of the man, including a forthrightness here that doesn’t always come easy, which makes McKellen all the more real and this documentary such essential viewing.

The actor sits in an armchair, dressed unexceptionally, as though he’s given scant attention to the cameras. Stephenson illustrates his recollections with a variety of visual material: family photos, archive film, clips from the actor’s filmed theatre performances and, of course movies; added to which the director has made lovely recreations of McKellen’s youthful memories – particularly of the youngster’s first enthralled visits to theatres, where he wandered backstage and immersed himself in the thespian world.

As a young boy in Wigan in the Forties and Fifties, McKellen’s love of theatre became his “compensation” for having no interest in girls. “Sexually I was blind,” he says. “Nothing to see, nothing to hear, nothing to feel. Making me feel in a sense that I did not exist.”

At Cambridge University, where he appeared in 23 plays in three years, he both confirmed his desired profession and was able to explore his sexuality, though it was still deemed illegal at the time. He suggests that his flaw as an actor is to truly let go emotionally, which can only be viewed alongside a life in which he was keeping his true self under wraps.

Stephenson continues to weave the professional and the personal: the loss of McKellen’s parents, his rise in the theatre (notably through some dazzling Shakespearean performances), the belated but successful film career, how the outbreak of AIDS and the Thatcher government’s introduction of the homophobic Clause 28 finally encouraged him to come out and, more, become a gay rights activist.

An embarrassment of riches includes McKellen’s modest yet inspiring thoughts about his craft, and a complex picture of how a gay person of his generation dealt with that part of his identity. A striking example involves his embarrassed admission of seeking to underplay the homosexual themes of the play Bent in the late Seventies, despite starring in it and being homosexual himself.

In contrast, there’s his pride now in the fact that an openly gay man can be cast as both Magneto and Gandalf, in blockbuster enterprises. “No-one said, ‘You can’t have Gandalf the Gay’. Though I didn’t play him gay. He’s 7,000 years old. I think sex is a thing of the past for him.”