In the film, the super sleuth created by Arthur Conan Doyle is well into his 90s and is in retirement, living out a self-imposed exile away from London.

For McKellen, Holmes' age and frailty proved part of the attraction of the role. "It was a story I could relate to," he says, "not because I am a great follower of Conan Doyle. I am 76, so the old age and mortality rang true. At the same time, it was all a bit of a lark."

Part of the story is told in flashback. "I get to play old and then also very old. That's fun."

Since this interview, the Conan Doyle estate has launched a lawsuit against the film-makers, claiming that the movie draws on the Scottish novelist's later stories, which are still under copyright in the United States. It has filed a suit against the movie studio Miramax, publisher Penguin Random House and author Cullin, who is also one of the film's screenwriters.

The fictional detective remains perennially popular. He is the star of a BBC television series, set in modern-day Britain, starring Benedict Cumberbatch as Holmes and Martin Freeman as Watson, and of CBS show Elementary set in contemporary New York with Jonny Lee Miller and Lucy Liu.

Holmes is also at the centre of the 2009 and 2011 pair of films directed by Guy Ritchie with Robert Downey Jr and Jude Law as Holmes and Watson, with Downey Jr earning a Golden Globe for his performance.

McKellen believes that audiences are always intrigued by detectives' private lives, especially that of Holmes, who is an opium addict.

"Conan Doyle may have started that but Agatha Christie with Miss Marple and Poirot followed on," he says. "There have been endless books about detectives and their personal problems, which may be at odds with their public image. That is certainly true of Sherlock and I think that's why people go back again and again to Holmes."

In the new film, Holmes is living in retirement in the south of England, where he keeps bees and is looked after by his housekeeper (played by Laura Linney), who has a son. "The story creeps up on you and then gets more complicated as it unfolds," McKellen says, describing it as "a mystery, a thriller".

Despite all the plaudits he has received on stage and having earned four Baftas and five Emmy nominations, he concedes that he will likely be best remembered for his fantasy blockbusters and his 2005 turn on the popular British soap Coronation Street.

"That's only natural," he says. "Some people have seen The Lord Of The Rings 20 times, so one becomes part of their lives. When I die, the headlines will be 'Gandalf dies', of course. That's fine because I am not ashamed of The Lord Of The Rings."

Citing the example of Alec Guinness, who played Obi-Wan Kenobi in the original Star Wars trilogy, he adds: "He hated being famous for having done Star Wars, but that was Star Wars. I did Tolkien.

"There are very, very few plays or films I have not thought were worth making. It would be awful to get stuck with a character you didn't think was very good.

"I am not a snob at all. There are some people who think I am a Coronation Street actor. Well, what a great accolade to have," says McKellen, who was knighted in 1991 for his service to the dramatic arts and received his second Oscar nomination in 2002 for The Lord Of The Rings: The Fellowship Of The Ring.

Indeed, if he were a snob, he would not be working with Condon for a third time, appearing as Cogsworth, the butler-cum-clock, in Disney's latest re-imagining of Beauty And The Beast. The rest of the illustrious cast includes Emma Watson, Emma Thompson, Ewan McGregor, Josh Gad and Kevin Kline.

"The cheek of it made me want to do it," he says of the film, which is a live action reinterpretation of the 1991 Disney animation. "They had written this fantastic character. He is a butler that gets turned into a clock. It is based on the Disney animation, although here, animation is used only for the characters that the prince sees when he is under the spell.

"Basically, I just have to supply a voice and the animators will do the rest. Then, at the end of the film, the characters all turn back into their real selves and then I sing and dance in a Disney movie."

He is also back on the small screen in many countries, starring alongside his old friend Derek Jacobi in the second series of the sitcom Vicious, about an old gay couple. This time around, McKellen says he feels more comfortable with the show.

"The history of sitcoms is that they get better and sometimes change in their tone. Also, I hope I have learnt a new level of performance. The problem I had in the first series was that they give us an audience of 500 people, which is quite a big audience, and they are very close in.

"In front of them are the cameras. And I was playing to the 500 people. My wily co-stars Marcia Warren and Frances de la Tour were playing to the camera. I think I have corrected that."

More TV work for him comes in the small-screen adaptation of The Dresser with fellow thespian Anthony Hopkins, a quintessential story about life on the stage, and he will also lend his voice to the animated film Animal Crackers.

He is still very much in demand and always happy to take risks, he says, something which he learnt from celebrated theatre director Tyrone Guthrie.

"Guthrie taught me to dare and to jump off the end of the pier and see if you can swim, and to demand things of yourself that you don't know you can do," he says.

This is contrary to his naturally reserved demeanour. "It is just the way I am. I was very shy as a boy. I think that was a lot to do with growing up gay at a time when it was illegal to be gay, so you didn't push yourself forward.

"You were always disguising. The great thing about coming out for me, professionally, was I had spent the first half of my career in disguise."

Acting, he says, was often a pretence, a way of hiding his true self. "When I came out, I was dealing with my own real emotions and acting came about as revelation and telling the truth rather than disguising things."

• Mr Holmes opens in Singapore tomorrow.