If you have tackled Hamlet at the start of your career, you will want to tick off Lear before you retire. The part has been played by Sir John Gielgud, Sir Laurence Olivier, Brian Cox, Christopher Plummer, Sir Ian McKellen - a litany of greats now joined by Sir Derek Jacobi in a thrilling production by London's Donmar theatre that tours to Glasgow this spring. This mythic character, who bequeaths his kingdom to his daughters only to be abandoned and hurled into madness, is a special part and Jacobi is giving it a special performance. "Overwhelmingly moving," raved the Guardian. "The finest and most searching Lear I have ever seen," said the Telegraph. These are sentiments confirmed by Jacobi's fellow performers when I meet them before the show. "He is an amazing actor and an amazing man," says Gina McKee, star of Our Friends In The North, who gives a chilling performance as the wicked daughter Goneril. "He's absolutely stunning as Lear." Sitting next to her in the empty theatre is Ron Cook, who plays the Fool. He is only in the first half and has a vivid memory of seeing the production all the way through for the first time. "I saw three runs and I was in bits," he says. "The first time I saw it I couldn't talk. I thought, 'Next time I see it I'll be fine,' but it was the same thing. It touches something very deep." Jacobi cuts a modest figure as he sits in a corner of the small Donmar bar. Even with the wild white beard he has grown for the part, he does not seem his 72 years. He does, however, have to preserve his energy - and, crucially, his voice - to power him through eight performances a week until June. Since appearing as a teenage Hamlet on the Edinburgh Fringe in 1955, he has established himself as a towering force of classical acting in leading roles such as Peer Gynt, Prospero, Richard III and Macbeth, not to mention I Claudius on TV. Despite this pedigree, in person he is genial and unkingly, a man with no illusions about the magnitude of the task before him. "I'm bearing up," he says cheerfully. "It's a wonderful play, with wonderful parts, but it's a mountain. Michael Grandage, the director, was the motor for the whole thing, telling me I had to do Lear. I kept saying, 'Give me time. I've got to get a bit older.' But this year coincided with my 50th year in the business and age no longer seemed a problem. I felt righter for the part than I'd ever done before." Certainly, it's on a different scale to his Emmy Award-winning guest appearance in the 2001 episode of Frasier in which he played an appalling Shakespearean actor. The dramatic pause he inserted into Hamlet's "the rest is… silence" was hilarious. "Doing Shakespeare badly comes naturally to me," he says with a laugh. "Playing Lear was harder." Harder and longer in the planning. He and Grandage have been talking about the idea for seven years. You could argue Jacobi's preparation goes back even further. He saw his first King Lear in Hammersmith as a schoolboy in a production starring actor-manager Donald Wolfit and has caught a handful of stagings since, most recently the Rupert Goold version starring Pete Postlethwaite. "Actors are scavengers," he says. "We steal from one another. When you see another actor doing something rather marvellous, you put it in the bag for future reference. Not to suggest that one's own interpretation is an amalgam of everything one has seen - it's not - but the more performances you see, it does start to peel the onion away a bit." Performed in the tiny Donmar, with the wooden planks of Christopher Oram's timeless set surrounding actors and audience, Grandage's production will work well when it is broadcast to cinemas on 3 February as part of the NT Live programme - a month before it arrives for real at Glasgow's Theatre Royal. Fast and focused, it creates a terrifying sense of mounting chaos as the forces of good (the innocent daughter Cordelia) are banished and the forces of evil (the bastard Edmund played by Glasgow-born Alec Newman) take control. The cast's assessment of Jacobi's performance is no exaggeration. It is tremendous, charting Lear's decline from charming autocrat to babbling geriatric in a way that is deeply moving. Even before Jacobi and Grandage had worked together on Twelfth Night, The Tempest and Don Carlos, the actor quickly found they had a shared approach to the role. "We are on the same wavelength," he says. "It has to be true to yourself and it has to involve your own personality, your own look, sound, you - it's yours." This being the case, it helped both men to have approached the performance slowly. "To know seven years ago that an actor who wanted to tackle Lear was asking me whether I would like to tackle it with him was a great privilege," says Grandage. "As an actor, he was most interested in finding a world. We wanted to create somewhere that had a pagan feel to it without going into a Flintstone place - out of that came timber, something very rooted, something of the earth. Our discussions about the text were about the bit of Lear that we both felt was most interesting to us, which is the domestic tragedy at the heart of it, the way this family tears itself apart." Like Hamlet, it is a role that gets under an actor's skin, making him think about the relationship with his own father and, according to Jacobi, relationships in general. "It's a very emotional part," says the actor. "He has to rage with incredible anger and incredible grief. Compared with Hamlet, I found Lear a harder nut to crack. It has to encompass a more hazardous journey than Hamlet's." The challenge as a performer is to feel those intense emotions every night, eight nights a week. Does he always feel them afresh? "I do try to," he says, with characteristic modesty. "Gielgud's famous remark when he was doing eight Hamlets a week was, 'Sometimes I have to send on technique.' I try not to do that. I try to immerse myself as much as I can." King Lear, Theatre Royal, Glasgow, 7-12 March; NT Live, selected cinemas, 3 February. This article was first published in Scotland On Sunday, 2 January, 2011