Within seconds the feuding duo are dragged apart, witness statements taken and relative calm restored for the more sober drinkers.
It happens every night of the week in pubs and clubs across the UK. A few years ago the police would have been called to sort out the fracas - now, any trouble is dealt with by bouncers.
Researchers at Durham University have studied nightlife in Britain's major cities and, in a report to be published next year, will conclude that bouncers now constitute a second police force. In places such as Manchester and Leeds there are as many, if not more, bouncers on the street after dark than police officers.
Indeed, private security firms are taking advantage of the fact that police forces are finding it increasingly difficult to recruit. This year alone the number of officers serving in the Metropolitan Police fell to 24,000 from 26,000 last year.
As the police move off the streets, because there are not enough officers to patrol them, so the number of bouncers increases. In the next couple of years more than a quarter of a million men and women will be employed as "private security staff". On Friday the Government announced details of an official training scheme for bouncers.
Cassandra is one of a growing number of female bouncers. Before getting her job she was vetted by the Nottingham police to ensure that she didn't have a criminal record. When her record was pronounced "clean" she was given an official badge and now works at Squares, an upmarket bar in the city centre, where customers wearing trainers are politely turned away.
At weekends, she is one of eight staff who patrol the club, on the look out for under-age drinkers. She regularly monitors the ladies toilets for drug-takers. Her worst experience was when a man pulled a knife on her. Colleagues quickly came to her assistance and her assailant is now serving three years in prison.
Nottingham is already piloting a compulsory vetting procedure for door staff in a city where 95 per cent of bars and clubs use private security. Currently there are well over 1,000 bouncers at work compared with 1,113 police officers on the street. By 2001 the bouncers will be in the majority.
Cassandra's view is that security work is a more attractive proposition than joining the police. She said: "You are faced with difficult situations, but the career prospects are far better than with the police and you don't have to do all the horrible work that the police have to, like telling people their kid has been murdered."
Outside Media, a fashionable drinking spot, Richard Williams is weeding out possible drug-takers and trouble-seekers.
His eyes scan the queue of goose-pimpled girls, whose bare feet are turning blue inside their strappy high heels, and the anxious young men in bicep- hugging T-shirts.
In a doorway, one man is noisily parting company with the copious contents of his stomach.
A martial arts teacher by day, Richard has been a bouncer for 18 years. With his chunky gold chains and black uniform he radiates menace, but he stresses that violence is never an option.
He says would like door staff to have more powers comparable with those of the police.
"The police are working with us a lot more now, but they used to think we were taking their jobs away," he says. "We now do incident reports if someone is escorted out and everything is captured on CCTV. There is also the option of citizen's arrest but we are still limited. The police have got batons - we should have more powers."
A gaggle of whey-faced boys in bow ties has reached the front of the queue but they have been drinking too much and are refused entry. Humiliated, they start shouting racist obscenities at the door staff. Another man sporting two ear-rings and a crew-cut starts taunting one bouncer, lunging forward to kiss him on the lips. The level of abuse escalates but Richard and his colleagues do not flinch.
Eventually the rejected trouble-makers shuffle off into the night, their insults getting bolder as they retreat.