Pay your pounds 1-a-day membership fee for the long, narrow room at the top of the building and Diana Quick may press your flesh as she squeezes past. The beautiful young actress from the radical theatre company, who a few hours earlier was a soulfully romantic Vita Sackville-West, shouts raucously: 'I've found a party.'
Illusions can be shattered here. The star bar is a green room on speed. Plus promoters; plus producers; plus press agents. It's networking in T-shirts and jeans,
but it's still networking. So many captive audiences.
'After a day's working at the Assembly Rooms, I come into the Assembly Rooms bar to work,' says Sharon Keane, an agent. 'After that, I might go somewhere else to have a drink.'
The bubble of fire that is the Perrier Awards chairman, Nica Burns, once of the comedy trio Fascinating Aida, is present, working, storming. The star bar is sponsored this year by Stella Artois and does not serve Perrier water. She leaves a Scottish barman verbally flattened.
At the top of the room, beaming like a benign head prefect, stands the ever-youthful-looking William Burdett-Coutts, artistic director of the Assembly Rooms, long the premier fringe venue, also head of Granada TV arts, also a member of the celebrated banking family, also the next director of the Edinburgh Festival but three.
'We all know that merchant banker is rhyming slang,' mutters Mark Thomas, one of the Assembly Rooms acts, after Burdett- Coutts has incorrectly introduced him to a group of journalists. But Burdett-Coutts continues to beam.
However, the occupants of some of the tables look miserable. Their shoulders droop; they glower and sip at their beer. People pass by on the other side. They are stand-up comedians. Perhaps they fear that if they make a joke, a neighbour will steal it for his act. It has been known.
There is no collective noun for stand-ups. An anguish of comics, perhaps? I tell them a joke I have just heard in a play about Tommy Cooper. 'I went to the opticians and said I thought I was short- sighted. 'What's up there?' the optician asked. 'The sun,' I replied. 'Well, how far do you flipping well want to see?' '
They nod sagely. 'Yes, that's a good joke,' agrees one lugubriously. 'I think that joke exploits the situation nicely,' says another earnestly. No one smiles.
Perhaps they had come from the Fringe Club, a house of ill repute and no-holds- barred heckling. It's about the only festival night-spot where you see real-life Edinburgh residents. At pounds 17 membership for the festival, it's cheap enough for local people to take advantage of, and the curious Edinburgh fashion of going out in long-sleeved shirt and tie but no jacket is in evidence.
The ties soon come off, though. The Fringe Club is at the university students' union, and the clientele is largely of student age. The place resembles the night after finals at college. You tread ankle-deep in beer and broken glass.
Onward and upward. Since the early Sixties, the bar at the Traverse, Britain's first fringe theatre, has been the place for intellectuals to while away the small hours. This year the talk is of both the official festival and the fringe lacking exciting material.
The Traverse is in stylish new pounds 3m premises, a stone and glass drum with
a large atrium. The bar is striking: 40ft,
the longest in Scotland. But no beer
bottle is hurtled down it, Wild West saloon style. This is a designer bar: 'spider chairs', 'sausage sofas', all custom made in Barcelona.
An exhibition of contemporary Russian art hangs on the walls. No comedians here. Playwrights, actors and studious devotees of new writing; directors from the Filmhouse across the road; English literature postgraduates agonising over contemporary drama and the decline of the British cinema industry. Perhaps it is the only night-spot noise-free and sweat-free enough for indulging in a little romance. But then again, there are probably bigger problems to solve first.
These tormented souls would probably be refused entry to the Rave and the Mambo Club, the festival's newest night- spots. The former - driving disco music, dry ice and various Sixties aromas - is held in a converted city centre church. Local residents have successfully prevented live bands playing, so a forlorn drum kit sits alone on the stage while the party-goers sit together on the floor drinking and dreaming in the clouds of dry ice.
At the Mambo Club no one sits. Bars, restaurant and foyers at the Playhouse Studio Theatre are converted into dance floors for the Latin American salsa rhythms. No one sits, but some lie, on thoughtfully placed sofas at the edge of the floor. Most, though, are happy to dance on and on. Dance with a dark-eyed lady in a little piece of Cuba in Lothian and it's hard to remember there's a festival on at all.
So, onward and downward. Slightly the worse for wear by now, I find myself singing the amended Frank Sinatra of the feminist comic Jo Brand: 'I get too hungry for dinner at eight; I punch small children when my period's late.' It earns you the strangest looks from policemen, particularly when you're male.
The Gilded Balloon is where anyone still awake just before dawn ends up. It seems virtually a 24-hour live cabaret, the Late and Live show running from midnight till 4am. The heckling is good, but the acts are better. Frank Skinner destroys one heckler by saying: 'Some comics have a plant in the audience; I've got a vegetable.'
'When the room gets full, it's safety in numbers,' says Steve Coogan, the voice of John Major and Neil Kinnock on Spitting Image. 'The attitude of the crowd changes. It becomes dangerous. If they don't like you, they let you know. They might talk loudly or unselfconsciously. That's particularly true if you give away a lot of tickets. When people pay, they usually try to concentrate for half an hour or so.'
The small bar at the Gilded Balloon is a stark contrast to the Assembly Rooms. No food, tables or bright lights here. Instead, a sweaty, thronging mass spilling out into the foyer and on to the street; every other face a comic, big and small time.
Eddie Izzard, one of the hottest names on the circuit, is chatting freely about his transvestitism. 'People ask me why I wear women's dresses. But I keep telling them, they're not women's dresses. They're my dresses.'
At 4am at the Edinburgh Festival, it seems a wholly reasonable point of view.