'Badfinger' is at the Donmar Warehouse, London WC2 (0171-369 1732), to 22 Mar

Michael Sheen made his directorial debut with The Dresser. Which made him the Welsh dresser. His joke, not mine - but it's pertinent given his latest project, Badfinger, the Welsh entry in the "Four Corners" season at London's Donmar Warehouse. That makes it sound like the theatrical equivalent of the Eurovision Song Contest but there's no competition, just a selection of new plays from all parts of Britain; anyway, Swansea- born Simon Harris has more to write about than onomatopoeic jingles and sentimental goo. (Go on, write to me and defend Eurovision on artistic grounds, I dare you).

Sheen, Harris and Natasha Betteridge formed the theatre company Thin Language (an ironic reference to a Welsh term referring to the English language) in 1991, the year that Sheen left Rada to appear in Martin Sherman's When She Danced in the West End. (He graduated in the middle of rehearsals.) Since then, his career has been meteoric, playing Romeo and Jimmy Porter at the Royal Exchange, a (Welsh-toned) Peer Gynt directed by Ninagawa and, currently, a lethally malevolent Lenny in Roger Michell's powerful revival of The Homecoming at the National.

His sideways shift into directing is all Thelma Holt's fault. This indefatigable producer has a proven eye for talent, having spotted Matthew Warchus very early on. "She believed acting might not be enough for me and started me off. She was exactly right, not because acting isn't satisfying but because directing uses a different part of my brain." Directing has not only enabled Sheen to read scripts differently, with a clear overview rather than looking at a potential role, but the process has relaxed him as an actor. It's an incredibly valuable lesson to have learned: alert relaxation on stage is a crucial quality in an actor.

Sheen, however, isn't stopping there. He is also part of The Foundry, a production company dedicated to producing exciting new writing. They have already snapped up the rights to Never Land, the latest play by Phyllis Nagy. For the moment, though, Badfinger is occupying him. "It's an urban Welsh piece, not another rural play." He describes it as a funny, dark and surreal, un-soapboxy, state-of-the- nation play. "It's not just the non-Welsh but the Welsh themselves who don't get their culture reflected back to them. If you don't get to see your own culture, you end up hanging on to cliche."