The Quiz, Trafalgar Studios 2, London<br />Sisters, Gate Theatre, London

David Bradley is wonderfully funny and sympathetic in this new one-man show by Richard Crane that affords the reliable pleasure of watching a great actor playing a dodgy one. Here Bradley surfaces as a decrepit old thesp, named Henry Thaddeus Irving Binkie Macbeth, etc, etc (the fateful list goes on). His father was a bishop and his mother was an actress, so he has been a bit of a joke since conception. And now, years later, he has dwindled to that hapless comic cliché: the tour de force who is forced to tour.

With his monk's habit and long, cadaverous face, he looks like a cross between a figure from El Greco and a cartoon from The Beano as he totters tipsily onto the crucifix-dominated wooden stage and starts to light the candles that will determine the length of the performance. Confessing that he's had "a sherry" backstage , he prepares to launch into his solo turn as Dostoyevsky's "Grand Inquisitor", a show he has been doing night after night for 20 years.

But, on this occasion, Angelica, his indispensable and sorely abused stage manager, has finally washed her hands of him and gone AWOL. Left high and dry (and he has some epic "dries" with no one on prompt book), he decides that he has been liberated from the tyranny of the text. This is his "bungee jump moment without the bungee". During this free-fall, hilarious digressions about career and family crazily mirror the ideological divisions in Dostoyevsky between the returned Christ and the proto-totalitarian Grand Inquisitor, who argues that it is kinder to offer the illusion of hope than trouble mankind with freedom of choice.

The whisky-swigging actor has always fancied conking out on stage ("in my tights, on my hobby-horse, in the limelight") like Kean, Irving, Molière and Tommy Cooper. Will tonight be the night?

Bradley achieves a terrifically warm rapport with the audience as he lurches squiffily from rehearsed ad libs to Tommy Cooper impressions, rambling allusions to Sartre and Sinatra, grandiose and anachronistic comparisons (he likens his career to Christ in the desert without a map – "Mein Kampf – as Jesus said") and a précis of the Dostoyevsky that is genuinely illuminating because of the irreverent paraphrasing and the contemporary parallels in it.

Granted that it's intentionally chaotic, Crane's uneven monologue could keep better control of its jostling elements. But Bradley, with his hollow cheeks and humorously haunted expression, has the charisma and the comic timing needed fuse its furious philosophising and luvvie anecdotage. And he manages to rise to a mad magnificence as he envisages death-on-stage as the seamless passage from one lie to another.

The character calls the evening "the jazz version" of his normal show: "Dostoyevsky meets Dizzy Gillespie". At the Gate Theatre, the cast are performing jazz riffs of a kind on Three Sisters in Chris Goode's mannered 90-minute deconstruction of Chekhov's drama of frustrated yearning. The difference is that the improvisation here is genuine. Chance plays a part, with the roles assigned to the actors (one male, five female) by the drawing of lots or the spinning of a bottle. The broad four-act structure is retained but within that framework, the show is different every night. Identities are suddenly swapped with the exchange of military caps or green sashes. Characters can be portrayed by several performers simultaneously. Dialogue is overlapped and often sounds like concrete poetry. A rabbit lollops about.

I approve of deconstruction in principle. In Denmark, I once saw an excellent version that unpicked the politics of impotently imagining the future by re-enacting scenes from the perspective of the sisters in old age. But nothing comparably revealing happens in the current piece, which is stronger on neurotically looped and listless mood than on analysis.

Technically, Goode's production is remarkable as the crew adjust the lighting and sound to the needs of the moment. But, thematically, it's a disappointment.

'The Quiz' to 28 June (0871 297 5461); 'Sisters' To 5 July (020-7229 0706)