False modesty? Maybe. As well as being a revered member of Britain's theatrical aristocracy, he's an inveterate talk show guest, celebrated for being frank and funny, and an eloquent campaigner for gay rights.

Yet he says he always felt as if he were acting – offering up the side of his personality best suited to the occasion.

With this film, director Joe Stephenson, a longstanding friend, gets past all that, winkling out the person behind the performer in a wide-ranging interview complemented with photos and film from every stage of McKellen's life.

It goes much deeper than the hugely entertaining anecdotal festival that Roger Michell made recently with Tea with the Dames. A rumination on the joys and costs of a life devoted entirely to acting, it hums with a poignant sense of self-discovery, as if McKellen is crystallising things he's only half-known about his life's milestones and motivations.

It starts with memories of his Lancashire childhood, which Stephenson has fleshed out with re-enactments – a risky manoeuvre which could have destroyed the mood completely. But these dreamy sequences, shot in soft-focus black and white, are imbued with the spirit of an old Rank or Korda film. As he remembers his childhood trips to the theatre in the 1940s, we accompany him, with Milo Parker, his co-star in Bill Condon's Mr Holmes (2015), perfectly cast as his stagestruck young self.

He talks, too, of his sense of isolation as the boys he knew started getting interested in girls. The theatre became his comfort and the source of his identity. He was Ian, the boy passionate about putting on plays.

At Cambridge, he could happily acknowledge his homosexuality. University life kick-started his career as he honed his technique in big classical roles, learning to use his whole body in crafting every performance, starting with the walk. While his theatrical work flourished, however, it would be years before he succeeded in film. It was his leading role in Richard Loncraine's Richard III (1995) that did it. After that, the screen opened up to him.

By the end of the film, he, too, is convinced that Stephenson has revealed the real McKellen. He's also pleased about it. The film, he says, will make a great obituary. As to his legacy, he hopes, with a burst of real modesty, that he's inspired a few young actors: "Those of us who don't have children need to feel that some part of our lives should be given to the next generation ...Then my living will not be in vain. That's about it, isn't it?"