Skylight, by David Hare, is at the Vaudeville Theatre, Strand, London WC2 (0171-836 9987) to 11 Oct

"If I could be reborn as anyone, I'm not sure Mary Poppins would be my first choice." Louche, rich restaurateur Tom is lambasting Kyra for her sacrificial goodness, her selfless life as a teacher to underprivileged children. His arguments are counterbalanced by what she sees as his appalling self-pity. After a three-year silence he has pitched up, demanding forgiveness for their adulterous affair which she ended the moment his wife discovered its existence.

Aside from a short overture and coda between Kyra and Tom's son, David Hare's gripping Skylight is taken up with the wistful, evasive, pungent, fiery reunion of the ex-lovers. The skill with which Hare knits together passionate arguments about love, guilt and responsibility, subtly dovetailing character and ideas within the play's overall arc, is enormously impressive. Hare's consummate craftsmanship is matched by Richard Eyre's exacting, beautifully paced production, now back in the West End with a new cast.

Because the cast of the first production made such a defiant, definite impression, recasting is particularly difficult. Do you try for carbon copies or seek out actors with new qualities? With Stella Gonet and Bill Nighy in the parts originally played by Lia Williams and Michael Gambon, Eyre has wisely opted for the latter.

Unlike the strung-out intensity of Lia Williams, Gonet brings solidity to the role of the idealistic teacher who denies her feelings. She is also a little older, which gives an interesting edge to her character's naivete. It's her best work for some time. Physically, the lanky, languid Nighy couldn't be further from the lumbering, bear-like Gambon. Nighy tends to trade on charm (he was a tremendous Trigorin, completely feckless but winningly attractive, in the National's earthbound The Seagull a couple of years ago), but not here. Where Gambon used weight, Nighy goes for a restless, rangy energy. He also drops well-rounded vowels in favour of a nasal accent which gives you the amusing impression of watching Nighy play Adam Faith playing the role. He finds much more comedy in what is, at times, an extremely funny play.

Then, just when you've been admiring the balance, something peculiarly unsettling happens. At the curtain call, Nighy takes precedence. Why? In terms of billing and fame, he and Gonet must be equals. In dramatic terms, she's barely off-stage. Surely this is a double act? Is the seeming balance really a sham? Nagging doubts you've had about her character's powerlessness resurface and the suspicion grows. Is it the man's play after all?