THEATRE / A close eye on the company they keep: The last thing David Thacker wants is a lot of bums on seats. Sarah Hemming joins the crowd at rehearsals for the RSC's promenade Julius Caesar

THE FIRST thing you notice in the RSC rehearsal room in Clapham, south London is the noise. Rehearsals are usually accompanied by a quiet hum; this one sounds more like an indoor swimming pool at peak time. While there are often a few extra people hanging around, here there is a crowd of at least 150. And most of the denim and leather- clad bodies belong not to actors, but to an invited audience.

Actors and audiences are like the bride and groom of old - they never meet until the proper time. The RSC cast for Julius Caesar is having an early encounter mid-rehearsal, however, because if they don't, they might trample their first audience underfoot when the proper time comes. This Julius Caesar is a promenade production - the audience move around the players on the open stage like golf enthusiasts moving from hole to hole. Early rehearsals have relied on the actors' imagination to people the room with spectators, but there comes a time when they need the real thing: 120 unpredictable bodies around whom they have to act.

The first 'audience' consists of 120 16- year-olds from two schools, Graveney and Southfields. Some are doing Shakespeare for A Level, some have never stepped inside a theatre, but the teachers feel the day will be useful for their pupils. 'We'll be doing something on the way crowds are used in the play,' says one teacher, Linda Callow, 'and how the day has been an act of crowd manipulation.' She glances around the room. 'I think they're loosening up a bit; at first they weren't sure of their role, when to move and how much.'

To the untrained eye the students don't look so very loosened up. Huddled in conspiratorial groups on the floor and against the walls, they look a challenge. It's hard to see how the actors, almost indistinguishable from the spectators in jeans and trainers, are going to invigorate them.

David Thacker, the director, arrives. Apparently unfazed by the sea of bodies before him, he strides to the middle of the room and announces the start of the afternoon session. Suddenly actors and audience polarise. Seizing tables and chairs the actors cut a swathe through the bodies on the floor, set up their scene and launch into the opening of Act 4. The students, flickering into life, shuffle towards the table to watch.

'This is a play about people manipulating or dealing with crowds,' Thacker had told me earlier, explaining why he chose this style of performance. 'I thought it would be theatrically exciting to have people there on stage so they experience being in a large group of people hearing the speeches. I hope it might make them think 'Would I believe this?' in a more concrete way than usual.' It also gives him an extra headache: having the audience on stage means he has to direct them, as well as the actors. 'They're not meant to be actually part of the action, but yes, if we want them to stand, we've got somehow to get them to stand.'

In practice this crowd control relies heavily on the cast, who have to spontaneously 'block' each performance as well as carry the story forward. During today's rehearsal the actors have already learned to shift a seated audience by striding purposefully across the stage, colonising their next bit of territory and trusting that the bodies will scatter before them. Getting a seated audience to stand up proves more tricky. Sometimes the audience seem reluctant to follow the action until the scene is well under way, and during a break the students explain their reticence. 'You're thinking shall I, or shan't I, and you chicken out at the end,' says one, while another adds, 'Maybe they should have a couple of actors in the crowd who could lead it.'

Suggestions like this are like gold-dust to the cast, many of whom are having their first taste of promming. A hearty debate evolves. Should the cast keep the audience flowing by moving among them, nudging them even - or will the audience feel shoved about? Should ushers be placed as 'plants' to make the first move towards the site of a new scene?

'The most useful thing about today has been that we've realised that the audience has to be made to feel very comfortable,' says Barry Lynch, who plays Antony. 'It's very useful, but it's tough, because you're still dealing with the text and it's hard to concentrate when you're surrounded. But I think today is the hardest it's going to be.'

Kenn Sabberton, who plays Casca, agrees: 'I found the whole thing a bit nerve-racking. It's so difficult in rehearsals to imagine there's 150 people there. I suppose we felt that they would move out of the way wherever we went. We realise now that we've got to decide where to move them to - is it better for them to be in the middle and to play around them, or do we really want to have the centre of the stage at some points, in which case, how do we get them out of the way? Once people are sitting, it's not easy to get them up. But it's also very difficult if they stay standing, because we've got nothing to raise us up. I came in this morning and I said to David, 'We should be playing this whole thing on stilts because as soon as we go in there, we're going to disappear.' '

Once the students have departed, the cast sit down to a post-mortem and discover that the day has raised far more questions than it has answered. Julius Caesar is a lengthy play and some of the scenes are up to 20 minutes long - but if they sit all the time, Thacker might end up with the static production he was trying to avoid. And while many of the actors welcome the excitement of unpredictable staging, they point out that fluidity is not desirable for the entire play - set pieces, such as Caesar's stabbing and the big battle scenes, need to be carefully choreographed. This means subtly shifting the audience to where you want them.

At the end of the rehearsal, many of the students said they would like eye contact from the actors and would like to be addressed directly. In previous rehearsals, Thacker had instructed the actors to avoid these tactics.

'I rather changed my mind about that today,' he admits. 'I'd been thinking, until today, that when actors spoke soliloquies they'd talk to the audience directly, but when they were characters addressing the crowd they would appear to address the whole audience but in fact would find eye- contact with fellow actors in the crowd. Today actually led me to change my mind.'

Kenn Sabberton raises a further problem: 'What happens if you challenge someone? If you address them saying 'Hence, home you idle creatures]' what do they do? Should they walk off? If they don't, you look a bit of a twit.' (And if they did the play would be over in minutes, as that line comes right at the beginning of the play.)

A straw-poll of the teenagers reveals that most would now book for a promenade ticket rather than a conventional one. This gives some encouragement to the actors as they leave, dazed and not a little confused by the proceedings of the day. Thacker, however, is in positive mood.

'One of the most important things I've got out of today is spacial awareness. Before today, I didn't really have a sense of how much space 125 people would take up, sitting on the floor. Also I've realised it requires the utmost clarity of thought and intention from the actors; you can't afford to be vague.'

'You can't cheat,' adds Kenn Sabberton. 'You can't do something lazily when they're that close. And heaven help you if you fart] Then you're really in trouble.'

'Julius Caesar' previews tonight and opens 5 Aug at The Other Place, Stratford-upon-Avon (0789 295623)

(Photograph omitted)