The sweet sound of the ovation

Five minutes before the end of the bio-musical, Marlene, the producers must have heard the sweetest sound in the theatre. It wasn't the voice of Sian Phillips, impressing us (as she was) with her imitation of Marlene Dietrich doing her cover version of Edith Piaf. Nor was it the opening chords of the next song, "Falling In Love Again". It was more of a dull clatter: a sort of thud-thud-thud-thud-thud-thud-thud that rippled across the stalls.

The first-night crowd had been applauding Sian Phillips, or Marlene Dietrich, or rather this strange hyphenate, this Sian-Marlene person, who had been standing up there, supposedly at a Paris theatre in the Seventies, to which the audience had been willingly transported, to hear first-rate renditions of "Where Have All the Flowers Gone" and "La Vie en Rose".

Past and present, reality and artifice, actors and audience: in Sean Mathias's canny production the night had become a theatrical blur. The usherettes, the real-life ones at the Lyric, had run to the front of the stage with bouquets, as they had been instructed to earlier on by Sian- Marlene. She hadn't seen the usherettes at first (as she had said she wouldn't), and looked thrilled when she caught sight of the flowers, which she had already told us she would need to put in water, so she could use them all week. The audience responded with glee to this mix of personalities: the Prussian singer and the Welsh actress. Five minutes before the end they had given her - OK, them - a standing ovation.

Sian-Marlene stood there, as indomitable as the Statue of Liberty, her sequinned dress bouncing off more light than a disco ball. She lowered her head and back down low, at right angles to her hips. This may have been Marlene's trademark approach for milking applause: staring at the floor as if looking for a contact lens. Or Sian may have developed this one for herself. But then who was meant to be lapping up the applause at this stage anyway?

Ever since Sian-Marlene made her first entrance two hours before at the side of the stalls, sweeping in with sunglasses and big voice ("Oh Philippe, for god's sake ..."), and then leaving enough time for a round of applause as she walked on stage, the principal dramatic question had established itself. Who exactly is the star - Dietrich or Phillips?

Sian was putting on quite a performance as Marlene, but then Marlene was putting on quite a performance as herself. The author, Pam Gems, who wrote Piaf and adapted Dietrich's 1930 movie The Blue Angel for the stage, has settled back into comfortable territory here with this exercise in bespoke playwrighting. She has tailored a script that fits Sian Phillips as closely as Terry Parsons's costumes. It's subtitled A Tribute to Dietrich. It feels like one to Phillips. There are two other members of the cast: a lesbian playwright Vivian Hoffman (a fresh-faced Lou Gish) who acts as her PA and gets one kiss with the star, and Mutti (played by the director's mother, Billy Mathias, making her stage debut at 78), who hasn't spoken a word since Dachau. As Dame Edna has shown, everyone needs a Madge in the corner to pick on: Gems might have created these two simply to keep Ms Phillips's spirits up during the quieter patches of the West End run.

This enjoyable if piecemeal script eventually throws up its hands so that we can get on with the cabaret show. Before that, Gems comes up with the standard one-person show techniques for drawing out the Sian-Marlene character: the telephone call, the interview with invisible journalist, the "I remember ... ", and the monologue that threads its way through exhaustive stage business, as she hoovers the dressing room, puts out photos of Churchill and de Gaulle, and goes through "Lola" with the lighting guy.

She dispenses showbiz wisdom ("a star can't have husbands"), snipes at Greta Garbo and Bette Davis and shares her problems about remembering lines, her injured leg, the temptation of drugs. It helps if you imagine someone like the late Roy Plomley, sitting out front, chuckling at the anecdotes, poised to cue in the next song and then falling into a respectful silence as he hears about wartime Germany and how she was spat on as a traitor ("zat'z hard"). It helps, too, if you take along your Dietrich Lexicon, as the accent is pronounced. When she speaks about "Zen", "whisky" and "conflicts", it is "zen" as opposed to now, "whisky" as opposed to safe, and "conflicts" as opposed to Rice Krispies.

It was lucky there were so many actors in on the first night. The role of the audience is often underrated, but if Marlene is going to succeed, it needs them to get into character too. Which was why that thudding sound must have sounded so sweet. As Sian-Marlene introduced "Falling In Love Again" the audience sank back down, happy in the knowledge that, just as the Sian-Marlene could slip effortlessly and imperceptibly between speech and song, sometimes seeming to do both at the same time, they could slip with similar ease between London and Paris, the Seventies and the Nineties, Sian and Marlene. This was the sound of an audience - jumping up, sitting down, jumping up again - getting into a piece, a diva-tissement, that seemed at times to offer an opportunity for Lily Savage. Hundreds of seats that had just sprung up, flapped back down again. Thud-thud-thud- thud-thud-thud-thud. Only mine, and those of a few others who might have preferred Sian singing Marlene as Sian, stayed where they were.

Declan Hughes's Halloween Night, the third of the Four Corners season of new plays at the Donmar, locks us into a remote cottage in the West coast of Ireland with a bunch young dissatisfied Dubliners: one in PR, one an ex-journalist, one an ex-actress, and so on. The evening is themed: fancy dress, red food, red drink. The play is just as self-conscious. The backdrop is a copy of Gericault's Raft of the Medusa,which gets interpreted and re-interpreted. Yes, they are stranded figures, in need of some kind of galvanising event. Far too much is explained: "Something I need to tell you ..." and "Powerful stuff in this house tonight." Not really, no.

Tolstoy's rarely performed The Power of Darkness gets an ambitious revival in the round at the Orange Tree, directed by Sean Holmes, translated by Anthony Clark. The preview I saw was slow, and miscast in the key role. But the play itself is major: as good an argument as you can think of for a Child Protection Agency. I wanted the Ivanov team to get their hands on it.

'Marlene': Lyric, W1 (0171 494 5045). 'Halloween Night': Donmar, WC2 (0171 369 1732), to Sat. 'The Power of Darkness': Richmond Orange Tree (0181 940 3633), to 31 May.