Choreographed commuters cut some rug in London's Liverpool Station for T-Mobile.

On an evening in early February, thousands of people crammed into London's Liverpool Street Station to count down the seconds until 7pm. When the hour struck, they switched on their mp3 players and began to dance silently, though the cheering from the camera-phone equipped onlookers shattered the air. The flashmob event was organized over Facebook, but the inspiration, the anonymous 22-year-old organizer told CNN.com, was T-Mobile's spot "Life's For Sharing: Dance".

The original spot, which took place on January 15, shows a crowd "spontaneously" breaking into a choreographed dance to songs such as The Contours' "Do You Love Me" and Millie Small's "My Boy Lollipop". The throng steadily grows until ordinary, unsuspecting commuters are brought into the fun. That fans of the ad would go to such lengths to recreate the spot is a testament to what Saatchi & Saatchi, London and Partizan, London were trying to create - a moment that can be shared and passed on by the public.

"When something out of the ordinary happens, the first thing people do is pull out their phone and start recording it or phone a friend," explains art director Rick Dodds. "We wanted to create a memorable moment that got the audience involved."

In aiming to create that moment, the team wanted as much as possible to avoid the typical conventions of creating a memorable TV spot. The final look had to accurately reflect the immediacy of the live event.

"It got to the point where there was literally footage we couldn't use because [even though it was real] it looked like a setup," explains director Michael Gracey. "There was a guy who came in on crutches and in some evangelical, revival-meet style threw his crutches in the air and started dancing."

While spontaneous moments were abundant on the day of the shoot, the lead up to the event was an exercise in meticulously and secretly planned production.

The first challenge was getting the station authorities to agree to the shoot, which consisted of 350 dancers, plus the public, having a virtual disco right after rush hour. Partizan producer Russell Curtis had to attend hours of health and safety meetings and ensure that crowds on the day weren't over capacity, which would mean the shoot's immediate cancellation.

To avoid a potential crush of participants, the dancers were given false information on the day and time of the shoot so that word of the project wouldn't leak prematurely. The dancers themselves were cast after 10 sessions. Only 50 were professionals.

"The whole thing hinged on how much public reaction we got," says Dodds. "If we had 350 gorgeous, 20-year-old [trained] dancers it would be intimidating for people to join in."

The dancers were briefed to make the public their priority. They shouted out each planned move, choreographed by Ashley Wallen (Kylie Minogue, Mariah Carey), and if they sensed that onlookers wanted to join in they were encouraged to pull them into the dance. The decision to select music that was familiar and, where possible, included the dance moves in the lyrics, was also key.

"Early on [the client] made us change the last song," says Gracey. "The original was [Edwin Starr's] "War". We had everyone march, change direction and then march through each other. Visually, it looked unbelievable but we needed to choose songs that people knew the dances to."

Despite the planning and rehearsals, including two nights rehearsing at the station between midnight and 5am when it was closed, Gracey found that he had to be somewhat hands off and just let the live event happen. "You can't exactly go, 'That was amazing, but can you just turn around and face the camera this time'," he quips.

Stationed in the crew's secret headquarters - two shops on the upper concourse with blacked-out windows - Gracey watched the action unfold live. Eleven hidden cameras were stationed in the roof, in fake photo booths and concealed in personal items like handbags that allowed the camera crew to film the action.

Gracey organized a camera person and spotter scenario. If the spotter saw a key moment happening where a member of the public was getting involved, he or she would literally grab the camera person and swing them around to film the action. Each section of the station was organized into lettered quadrants. So as a member of the public was being filmed, runners would be dispatched to the quadrant to take a headshot and have the person sign a release form. In total, 1,000 releases were signed.

"We actually lost shots because we'd have a beautiful moment and in the foreground would come a runner with a digital camera in one hand and a handful of releases in another," says Gracey. Unfortunate as it was, the runners were the only way to ensure that releases were signed on the spot before the commuters went their respective ways.

Shoot over, the team immediately rushed to edit and complete the spot, which had to be delivered to the agency and client by 9am the next day for its 9pm broadcast debut. The insanely tight turnaround time was to avoid being beaten to the punch by commuters who filmed the event and would inevitably post it on YouTube.

"Me, [writer] Steve Howell, Michael and the two editors [freelance lead editor Diesel Schwarze and Final Cut, London editor Ben Harrex] were locked in a room working straight through the night," says Dodds.

"It was the best 40-hour work day I've ever had!" laughs Howell.

Saatchi & Saatchi, London http://www.saatchi.co.uk
Partizan http://www.partizan.com