Will & Grace star Sean Hayes gives the performance of the decade in Goodnight Oscar

1950s America is another country. They do things differently there. Oscar Levant was a gifted pianist and wit, a supreme interpreter of Gershwin and much in demand for live TV programmes such as The Tonight Show. In some ways he was a throwback to the likes of Dorothy Parker, Robert Benchley and the other wits and literary types who assembled at the Algonquin Round Table three decades earlier.

But Levant was different. His acidic humour was fuelled by Obsessive Compulsive Disorder, anxiety and possible schizophrenia that was barely held in check by industrial quantities of drugs. He was outrageous, unpredictable and brilliant but he was a liability to almost everyone who got close to him including his wife, his children and his best friend, Hollywood chat show host Jack Paar.

Doug Wright's play takes place one fateful night when Paar (Ben Rappaport) invites Levant (Sean Hayes) onto the first Californian episode of The Tonight Show. Levant's wife June (Rosalie Craig) arrives in advance, bearing good and bad news. Good: Oscar is on his way. Bad: he is on a four-hour furlough from a mental institution....

Although no one knows this aside from June and the nurse/minder Alvin (Daniel Adeosun) who has been press-ganged into accompanying him.

When Oscar stumbles through the door like a walking time bomb, it is clear that it's going to be a bumpy night. What follows is a fictionalized account of the green room lead-up to his appearance on TV. To say that Sean Hayes (Hollywood's Jack McFarland in TV's Will & Grace) gives the performance of the decade is no exaggeration. Not only does he immerse himself in Levant's complex personality, but he also exhibits a concert level talent for the piano in the second half without stepping out of character.

Having been warned by NBC boss Bob Sarnoff (Richard Katz) to avoid topics such as religion, politics and sex, Parr does exactly the opposite, nudging the microphone closer to his friend to get his responses to all three subjects.

In the course of a tight 100 minutes of theatre Wright's play expands beyond the subject to explore the origins of live television, American values, popular media and its attendant manipulation, mental health and the fine line between supportive friendship and exploitation.

The fantasy sequences involving the ghost of his mentor/tormentor George Gershwin (David Burnett) are chilling and there are trace echoes of Peter O'Toole's comedy My Favourite Year in the scenario. If the dialogue wasn't so screamingly funny and the performances so attractive, the portrait of a gifted man tearing himself apart with insecurity would be unbearable. But Hayes' Oscar is the magic key. On one hand, you can hardly bear to watch this fractured soul; on the other, you simply cannot tear your eyes away from him.

GOODNIGHT OSCAR: THE BARBICAN THEATRE TO SEPTEMBER 21