Quite who Prospero is, what he represents and how to convey his mysterious power are quandaries to challenge the greatest actor and director. Peter Brook, directing the play in 1990, chose an actor from Mali to suggest the character's enigmatic authority and familiarity with the invisible world. Adrian Noble, directing the RSC's new Tempest has stuck closer to home. His Prospero is David Calder - who lives in rather less magical Clapham.
At first glance, it might seem an odd choice. Calder does not possess the hawkish, ascetic looks you might associate with the part, nor that haughty authority. He has a pleasing face, a substantial frame and an approachable manner. He looks rather too nice, somehow. Yet anyone who has seen him act (and he has good-naturedly admitted that, after decades in the business, his face is still better known than his name) will be aware of the gravitas he brings to performance: a blend of intellectual precision and humanity. His Shylock - in David Thacker's 1993 RSC Merchant of Venice - was masterly, bringing out the dignity and pathos in the misunderstood Jew that are often missed.
Calder certainly feels the responsibility of the mantle he has taken on. Not only does he have to strike a bull's eye for his own sake, but also for that of his director. Noble has been working his way through Shakespeare's late great plays about redemption and reconciliation - The Tempest ought to be the jewel in the crown.
"When I first started re-reading the play, I almost thought, `I wonder why the hell I just said I would do this'," Calder admits. "It's awesome. It is the most extraordinary play he wrote. You feel that, in this play of all plays, the actor has somehow got to walk in the steps of Shakespeare; you really have to get into the man himself. You can't just think of a few ideas and an interpretation and then just bang it together. It needs constant analysis and unpeeling in the rehearsal room."
Calder clearly doesn't stop unpeeling at the weekend. We are talking in a smart brasserie near Clapham Common, surrounded by families eating upmarket scrambled eggs for Sunday brunch. But Calder is on the magic island, consumed by the play's complexities. Part of the challenge, he says, is that Prospero himself is on a rollercoaster.
"He starts by raising this storm which he says he's in control of. Then as soon as Ariel comes along, who has been doing all the work, he asks, `They are safe, aren't they?' So there is a sense that everything is uncertain - and that, I think, is the dramatic drive of the piece. You don't know that there will be some kind of reconciliation. It could all go wrong. Everyone is in jeopardy in that play and nine people's lives are changed beyond compare. But the person who goes through the most intense life- changing experience is Prospero... And forgiveness doesn't necessarily equal happiness, either. I mean, he's not a happy bunny at the end of it. It's a very disquieting piece."
It is, of course, the very ambivalence of The Tempest that has led many to try and pin it down. There are almost as many interpretations of it as there are layers within it. Does Prospero represent Shakespeare himself and the climactic breaking of his staff signify the writer's own withdrawal from the stage? Can Caliban be seen as the New World and Prospero as colonial power? For Calder, such interpretations are limiting.
"One thing I do feel is that it's not a play about colonialism. It's a far, far earlier stage than that. Caliban represents a sense of failure, if you like, on Prospero's part. It's more to do with Prospero risking his own humanity with Caliban, only to be disappointed, and therefore turning against him. I think that's the important thing in that relationship, not whether Prospero is striding round in jackboots.
"Prospero is a scientist and a looker into forbidden knowledge... You could make a contemporary link with somebody who not only established the principle of splitting the atom but actually built the bomb. The responsibility of knowledge is so alive in this play. That, for me, is how the politics express themselves; not in terms of reducing it to painting colours on the stage."
While he is cordial and generous company, Calder is a serious man. His eyes are warier than his manner, and, in talking about the play, you feel he is wrestling with an intellectual python. He finds a "strange melancholy" in Prospero and has mulled over, too, the significance of the character's "lone parent" status: "I sometimes wonder if losing his wife was what drove him into that library and into the inner world of the mind."
Dr Robert Bramwell, the television role Calder is probably best known for, is also a man with a precious daughter and no wife; so too is Shylock. Indeed, in his RSC performance as the Jew, Calder added to our understanding of the man in one simple stroke - by gazing poignantly at a picture of his deceased wife.
Calder acknowledges the coincidence in his chief characters' circumstances: "I seem to be cleaning up on these parts," he laughs. The only way of rooting a figure like Prospero, he adds, is to draw on his own experience.
"It does sound preposterous to say it, but with Prospero you actually have to find it in you. It is such an intensely personal journey that you can't be outside of the experience; you have to be entirely in the moment of Prospero. Therefore what you plug into is your own inner landscape."
Yet whatever the inner landscape that might help fuel his Prospero, Calder is not about to divulge it. He is an intensely private man and, faced with an opportunity for personal revelation, he draws back.
"There's an area I could talk about - but I'm not going to talk about... which is about a father-daughter relationship," he says, in a manner that invites no probing. He talks instead about the parallels between Shylock and Prospero.
In a sense, one could say that Calder has been tipped in at the deep end, as far as Shakespeare is concerned. How does he feel about leapfrogging over the juvenile leads into, shall we say, the more mature roles...
"You're being very polite," he says, laughing. "Well, it's wonderful, isn't it? They are the best parts. Of course, I would have loved to play Romeo - although, then again, I'd probably have been much more attracted to the difficult Mercutio. But, no, I'm still waiting for the romantic role. I mean, never say die. It's about time they made good, sexy, lively, real love stories about 50-year-olds."
In the meantime, Prospero will have to do. Calder is looking forward to submitting himself to the storm. "There is a way in which engaging in these pieces is energy-making as well as energy-draining. I used to feel terrific after The Merchant of Venice. Although it's emotionally very heavy, I used to come out feeling rather refreshed in a peculiar way.
"Mind you," he adds, "you do have to shed plays you know. You have to shake it off after you've done it. Because, in the end, you're not that person."
`The Tempest' previews from Thursday and opens on Wednesday 25 February at the Royal Shakespeare Theatre, Stratford-upon-Avon (booking: 01789 295623)