Manchester City manager Roberto Mancini appeals for calm ahead of second leg at Old Trafford cauldron.
Some of the most memorable atmospheres have a dark edge. Fixtures such as the Old Firm clash, England v Argentina, the Sheffield derby and Liverpool v Manchester United, among others, make great box-office drama partly because of the febrile backdrop.
But there's a balance, a fine line between fever pitch and frenzy. With the world watching tonight, Manchester must ensure it does not become a town called malice.
"Don't look back in anger!" Roberto Mancini, Manchester City's manager, rather wittily put it on Tuesday when asked for his pre-match message. "It's fantastic Oasis, yes?'' The mood music at Old Trafford tonight will be anything but an oasis of calm.
Like Sir Alex Ferguson, Mancini must urge his players to avoid any act, whether confronting an opponent, dissent towards officials, even badge-kissing, that might set the mercury rising higher in the stands.
The second leg of this Carling Cup semi-final could be a night memorable for the passion of the fans, for the skills of Wayne Rooney or Carlos Tévez, or an evening remembered for mayhem caused by a minority. It only takes a few to ruin the party for all.
The first leg at Eastlands was notable for the malevolent as well as the spectacular. Before kick-off at night games, City flick the switch, staging a light show straight from the old Hacienda nightclub. Last week, United fans reacted by holding up red flares.
The tone was set for an epic evening, the players clearly responding to the adrenalin rush surging out from the crowd. Few of the morning reports failed to mention the electric nature of the occasion.
Sadly, a small minority of fans lost control at Eastlands, throwing missiles inside the ground and engaging in skirmishes outside. Such criminal activity is being investigated by Greater Manchester Police, who have appealed for restraint tonight.
With additional stewarding and 75 extra police officers drafted in, a major security operation will take place outside Old Trafford with fans being searched for flares and missiles, although the vexed issue of coins remains.
How can loose change be confiscated? With many fans often arriving only shortly before kick-offs, knowing they have a guaranteed place in the all-seat era, it would be no surprise if the kick-off was delayed.
Whenever the game starts, both sets of footballers must ensure that any enmity that has built up over the Tévez-Gary Neville spat is not allowed to spill over into the game. The police have spoken to the clubs, and will remind the players again that they must avoid any action that provokes the fans. The atmosphere will be stoked up enough without the players fanning the flames.
"I think we have enough experience of playing in big games to know that [players must behave],'' Ferguson said. "It's always a key factor. With feisty games over the years, in terms of the competitive level of Liverpool and Leeds games since I've been here, the players' behaviour has always been good.
"They play within the rules of the game. We've never had any issues with big games. It's going to be a fantastic game, maybe even better than last week's, but we're ready for it.''
Mancini, commendably, called on all parties to keep everything in perspective. He said: "It's important that we play calm and it's only a game. It's an important game but only a football match. We want to win at Old Trafford and it's possible and we have a chance but we must play quiet and concentrate.''
The Football Association will hope so. The last thing the governing body needs is a high-profile inquest tomorrow at a time when it is bidding to host the 2018 World Cup.
In a week when Sepp Blatter, the president of Fifa is being entertained by the Spanish, the English can ill afford pictures of any trouble being beamed into Madrid hotel suites. Rarely has the Manchester derby carried such global significance.