Firth is lost for words as the monarch whose dilemma gripped the country

Watching from close quarters, I first hear Bertie's approach as he climbs the steps to reluctantly meet the expectant crowds. Top-hatted and wearing overcoat and gloves, he emerges, stony-faced, to greet an expectant public. He grips his speech tightly and stares fixedly ahead. As he struggles to get the words out, raindrops the size of sovereigns begin to spatter the sheets of paper in his hands.

"I have ... received ... from his Majesty the K-K-K... the King, the following gracious message. At ... the close of the British Empire Ex-ex-exhibition, I wish to express my thanks to you as P-..."

The tense silence of the watching throng becomes intimidating. Behind Bertie, Princess Elizabeth wills him on. In the spectator stands, ordinary folk bow their heads in embarrassment.

This dramatic moment launches The King's Speech, the historical drama – based on real-life events – that is set to lead Britain's chances at the Oscars in February. Starring Colin Firth as Prince Bertie, Duke of York and later King George VI, the film is a quirky buddy movie as the anxious royal looks to an unorthodox Australian speech therapist, Lionel Logue (Geoffrey Rush), to cure his debilitating nervous stammer.

Firth delivers his halting speech many times over one-and-a-half freezing days. It is December, 2009. The location: the (fake) fog-shrouded terraces of Leeds United's Elland Road ground. Around Firth and co-stars Helena Bonham Carter and Derek Jacobi, sit 250 extras in 1920s' costume. Glimpsed over his right shoulder is a policeman in helmet, cape and white gloves. That lonely rozzer is me, and I have a grandstand view.

Momentum on the picture has been building for weeks, ever since open auditions were advertised in local papers. Hundreds of people turn up for costume fittings. I am one of them. Initially hired as a soldier, I am re-costumed at the 11th hour as PC 752. Shooting days require an early start. I am up at 2.45am and arrive at Bradford Bulls' Odsal stadium an hour later. By 5am, 250 extras are queuing for a hot breakfast. Dignitaries rub shoulders with working men, vicars, soldiers and policemen. By 6am, we are on the road, heading for Elland Road. Filming begins promptly 90 minutes later on a 10-hour continuous day.

Those of us involved in the scenes in Leeds are delighted to discover that we are providing a backdrop to a visual effects sequence in which the young prince gives his first live radio address. The event is the closing ceremony of the British Empire Exhibition at Wembley, held on October 31, 1925. At its core is Colin Firth.

Prior to arrival, we are given a pep talk. The day has been split. Elland Road is being used for the speech elements of the prince stammering his way through his first public address. The Bulls' stadium was selected because it has curved ends, just like Wembley. Our collective role is to react to Bertie struggling through his speech to the assembled masses.

The experience is both quietly exhilarating and mildly harrowing. Firth mounts the steps, gulps and staggers through his words more than a dozen times. Excitement slowly gives way to feelings of concern for the prince, mixed with a desperate desire to stay warm in sub-zero temperatures.

Over time, a mood of stoic resilience spreads among the multitude. Everyone seems to be willing on Bertie/Firth. People have momentarily forgotten it's a movie; they just want the poor chap to succeed.

"I want everyone to bow their heads," says director Tom Hooper as he emerges from watching the scene on a monitor. "Remember, you are embarrassed at this dreadful performance from Colin Firth. I mean Prince Bertie." Hooper leaves the set, smiling mischievously.

"Not very helpful," mutters Firth as he prepares to re-play the speech for the umpteenth time. He glances wearily at the watching crowd and traipses below for a momentary blast of warmth.

"It's interesting, not so much what the stammer sounded like, it was how he struggled against it that interested me, how he tried to deal with it," says Firth. "When he hit one of those blocks when he was speaking publicly, you see him gathering himself, you see the attempt to calm himself, and that hesitation, feeling like an eternity with thousands, or in the case of radio broadcast, millions, of people hanging on every word.

"He really did draw the short straw in terms of what was going on in history at that moment. His father was the first King of England to have performed a live radio broadcast. Every previous king in history didn't have to worry about live radio, and every other future monarch would have the protection of edited and recorded footage. Bertie had to sit in front of a live microphone and speak to the whole Empire."

Between different camera set-ups Firth, Bonham Carter, Jacobi and various supporting actors huddle around hot-air blowers. Scalding tea is slurped out of plastic cups. Film stars mingle with extras. There is no formal divide; everyone mucks in.

Up above, the scene is re-dressed. Having filmed the upper tier, Hooper now prepares to repeat the process with the lower tier. Dubbed "Tommy Twelve Takes" by one watching wag, he assiduously covers every aspect of the sequence.

"Tom doesn't let anything happen just because it's the easy choice," says Firth. "As a result, what he gets on the screen is very richly textured; he never gives in to clichs.

"Tom is a fiercely intelligent and imaginative director who is fastidiously devoted to getting to the root of every problem the story poses. He will not give up until it's as authentic and as interesting as it can possibly be."

Of his star, Hooper responds: "Colin brings a wonderful specificity to the role; his body language, the way he speaks; he studied the way Bertie stammered very carefully. He has risen to the challenge and succeeded, as you really care for this man. This film stands or falls by the amount you care about his fate."

October, 2010. The King's Speech receives its European premiere at the 54th London Film Festival. Firth, Rush, Bonham Carter and Hooper are on hand to promote the film, which is already being talked of as a magnet for awards.

Says Firth: "The fact that people are talking that way is a sign of how positively they have responded to this, which is incredibly gratifying. People don't owe you their gratitude because you tried very hard (but] so far we are getting a lot of warmth."

A month or so later, I see the film at a Press preview. The movie begins with a key sequence: the closing ceremony of the British Empire Exhibition. My scene.

But, what's this? Two days of freezing discomfort amounts to less than two minutes on screen. Bertie's tortured speech has been truncated to a brief, dramatic prelude for what is to follow later. My moment of glory amounts to precisely... five seconds. Blink and you'll miss me. Such is the life of a lowly film extra.

But it all looks tremendous. The visual effects wizards have taken scenes of assembled soldiers, nurses, St John Ambulance personnel and cavalry at Osdal, and seamlessly transplanted them to Leeds. Scores of mannequins, used to bulk out the crowd at Wembley, can barely be separated from living, breathing background artistes.

And Colin Firth? He is everything Tom Hooper hoped he might be. When he finishes his final take of that first speech, the watching extras and crew burst into spontaneous applause.

He'll have to get used to that if he wins the Academy Award on February 27. Eight weeks and counting...

The King's Speech (12A) is released on January 7.

Yorkshire gets a starring role as film-makers put the county in the picture

The team behind The King's Speech aren't the only ones to have put Yorkshire in the spotlight. In the last 12 months, the county has appeared in a clutch of big- and small-screen productions, from comedy to dramas and big-budget movies, and had a starring role in one of the most eagerly-awaited films of the year.