Speed-the-Plow, Old Vic, London<br/>The Hour We Knew Nothing of Each Other, NT, Lyttelton, London<br/>Scarborough, Royal Court Upstairs, London

Kevin Spacey almost somersaults – tumbling through the office door, panting – as the lights snap on in Matthew Warchus' new staging of Speed-the-Plow. Obviously, David Mamet's sardonic portrait of avaricious film producers gains oomph when real Hollywood stars – Spacey and Jeff Goldblum – are playing Charlie and Bobby, the Philistine duo with a blockbuster film within their grasp.

However, what's remarkable is how far Warchus pushes this modern classic towards screwball comedy and physical clowning. Here, the two studio buddies are so overexcited at the prospect of megabucks that they can't stop farcically hopping and jigging, like small boys on a sugar high. Except that Charlie is taking something a little stronger than sherbet each time he dashes into the washroom.

Spacey is terrific at startling bits of stage business. Hilariously wired, he hurls himself to the floor, having an attack of sit-ups. And he's puffing, all the while, on a cigarette. Meanwhile Goldblum is impossibly tall and wonderfully kooky. His Bobby tries to act cool: sharp-suited, towering over Spacey, and swearing he'll screw the temp, Karen. But he's insecure and ludicrously spindly, with legs like a stick insect. His darting actions – licking his lips – also have a touch of cartoon locust.

These stars are a storming double act. Still, I have seen more chilling productions. In fact, so hastily is Act One presented that large sections of Mamet's patterned dialogue get swallowed in overlapping interruptions. I found this exhilarating then wearying. The play becomes rather a bore when Laura Michelle Kelly's Karen – supposedly on a mission to convert Bobby to a purer cause – bangs on about a rambling apocalyptic novel.

Nonetheless, the closing battle over the two film projects and Bobby's soul is thrillingly potent. Spacey is by now ferocious and desperate. The final victory is not just sardonic either. Which rival might have saved Bobby from himself remains ambiguous, a matter for debate.

In many ways The Hour We Knew Nothing of Each Other is a lovely antidote to dramatic conflict. This quietly absorbing piece, by the avant-gardist Peter Handke, dispenses with a host of theatrical conventions. In the course of an hour and a half, 450 characters simply cross a town square. There is no storyline. They just come and go, seemingly randomly, and no one exchanges a word.

This is at once strange and instantly familiar. It's as if you – or Handke or director James Macdonald – have been sitting on a bench just watching life passing: joggers, a skateboarder, a businessman chasing a pickpocket, girls giggling. It also grows surreal. There's a man with a cradle for a hat, Moses with a stone tablet, and a panic-stricken crowd beckoned by two figures in gorgeous tribal costumes who drift by in a gondola. It's as if the square is everywhere or everywhen.

Some vignettes are too caricatured, cute or clichéd. Moreover, you might feel that Handke – he created this in the 1990s – is not so much avant-garde as indebted to Cartier-Bresson, de Chirico, Ionesco and Jacques Lecoq.

Nonetheless, Hildegard Bechtler's cityscape of raw concrete buildings is eerie, somewhere between an architect's model and a bombsite, haunted by occasional apocalyptic sirens and gales. Meanwhile Macdonald's ensemble (Sarah Woodward, Justine Mitchell, Jason Thorpe and two dozen more) are fabulously mercurial and beautifully choreographed, creating an ebb and flow that feels eternal.

Finally, I left Scarborough knowing secrets about the characters but in a state of incomprehension. Warning: it is pretty much impossible to discuss Fiona Evans' Fringe First-winning play without giving away at least two dramatic surprises. However, let me begin by saying the opening half is enthrallingly intimate in this London premiere, directed by Deborah Bruce.

A laddy teenager called Daz (Jack O'Connell) is having a naughty weekend, in a seaside B&B, with his raunchy twenty-something lover, Lauren (Holly Atkins). They're having a laugh. She's doing a jokey reverse strip-tease, twirling her belt as she gets dressed. He's bouncing on the bed to wind up the landlady.

This is twice as engrossing because you're right there with them, as if theatre-going has turned into a game of voyeuristic sardines. Punters are perched on the windowsill and the bedside tables and, in my case, under the standard lamp – where Lauren's twirling belt almost tickled my nose. Both actors are superbly natural, comical and painfully confused. The revelation, and complex moral problem, is that she is his teacher.

What I don't understand is why we then have to sit through the same script all over again, with a teenage girl and a male schoolmaster. What sex they are makes little difference ethically, and the performances are less emotionally searching the second time around. Why bother?

'Speed-the-Plow" (0870-060 6628) to 26 April; 'The Hour We Knew Nothing of Each Other' (020-7452 3000) to 5 March; 'Scarborough' (020-7565 5000) to 15 March