Are you designing the costumes for Caligula, and do you always produce the clothes as well as the set for productions you're involved with? "Yes, and yes! The designer's job is to create the environment in which the director's vision of the play is realized, and for me, this means the costumes as well as the furniture."

Coming up with authentic costumes for plays set in the ancient world is notoriously difficult, which is perhaps why so many productions plump for modern dress. How have you approached the costumes for Caligula? "They aren't modern at all, but then they aren't a sort of ‘I, Claudius’ seventies television style either. The costumes the cast wear are representative rather than an imitation of what would have been worn in those days, because if I had put them in togas and so on, then such a literal re-creation of the clothes would have necessitated a literal recreation of the sort of monumental architecture that everyone associates with Ancient Rome — and especially with the palaces of the Emperors.

"This then becomes an issue of practicality as well as personal taste. The Donmar is a wonderful acting space, which is one reason it’s been so successful, but it's also — and that's part of its charm — a small space, too. So trying to build a couple of pillars, say, on the stage, of an appropriate scale to people wearing togas would have dwarfed the actors and looked ridiculous, as well as leaving no room for people to move around!"

Does this mean we get a fairly bare stage? "Not really, but it isn't cluttered. Although the Donmar is very well-known, the design budget — as with most theatres — is fairly . . . economic shall we say . . . so one of the challenges that faces any designer is creating something that looks good and helps the play work, while not costing a fortune. A good example of this is how we came up for a solution about representing Caligula's self-obsession: mirrors play a large part in the design, and it would look spectacular for him to shatter a large mirror every night in a climactic dramatic moment. But that would also be way beyond our budget. So I use a pool — which also has obvious associations with the Ancient Greek myth of Narcissus, the young man who fell in love with his own image — and Caligula can shatter his own image by striking the water. Great image, small cost!" You created some wonderful sets and costumes for The Vortex, which was Michael Grandage's inaugural production at the Donmar. The sets, though authentic, looked very modern? "1920’s furniture and design was often very 'clean' and simple so it has the effect of looking very modern and — which is important with many Coward plays — chic. We found some very good shops locally — one workshop was in Drury Lane! — and most people like the results. Critics can be hard to please, though, and someone had a go at Privates on Parade, which I designed at the Donmar in 2002, even though we had lots of scenery, interior and exterior, created an army dormitory on stage as well as a hotel room for the lovers' scene, and went from army bases to a camp theatre review where Roger Allam had plenty of costume changes, to a dockside scene as the troops prepared to head home. . . . Most designers have an ambivalent attitude towards critics anyway, as all too often designers only get mentioned in reviews if the critic doesn't like something about the set or the clothes."

There's a lot written about how to be an actor, but not much advice on careers in design. What's the best way of getting started? "Basically it’s down to job experience. You can study at art college, which is a help, but working on actual productions on the fringe is the best way in, as well as teaching you valuable lessons about the need to achieve maximum effects at minimum cost. I did a lot of very enjoyable work at places like The Old Red Lion, the Finborough — where I worked with Mark Ravenhill — and the Drill Hall. After a while you can get to the fortunate position, where I am now, of having an assistant, but at the start you have to do everything yourself, which is the best way to learn any job. The other lesson is that you have to have several freelance contracts on the go or in the pipeline at any one time to make a living. But it's a very enjoyable one!"

Caligula, starring Martin Sheen, translated by David Greig, directed by Michael Grandage and designed by Christopher Oram, opens at the Donmar Warehouse April 30, and runs to June 14.