Nobody expected theatre relayed to cinemas would work – until National Theatre Live became a hit. Matt Trueman watches one show come together

Gunnar Cauthery is building up to his big moment. He's on stage at theNational Theatre, playing a Conservative MP in a drama called This House. Despite hoping for a safe home-counties seat with haystacks and hillocks, his character has just been elected MP for the Worcestershire town of Redditch. "You can't find a haystack in Redditch," he bellows, "because of all the f****** needles!"

Ninety-five miles away, in Redditch, a huge cheer goes up at the local Vue cinema. In Sheffield, there's applause when another Tory MP snootily suggests Manchester "either needs a good clean or a good fire", while at the Covent Garden Odeon in London, the man next to me almost chokes on his noodles from laughing so hard. "I hope people cheered like a football crowd when they heard their town mentioned," playwright James Graham says the next morning. "I like to imagine a Mexican wave of laughter going round the country."

Not just this country. Director Jeremy Herrin has a friend watching in New York. One cast member receives a congratulatory text from Canada. Graham gets tweets from strangers in Amsterdam, Berlin and Bucharest. His mum's watching in Mansfield, Nottinghamshire. "That people have invested time, energy and belief that my play would find a global audience," he wonders aloud, sipping his hangover-beating coffee, "I've got no sense of the scale of it."

On 16 May, the NT Live broadcast of This House played to 45,000 people in cinemas around the UK. That's equivalent to selling out the National's main auditorium for more than five weeks solid. Another 20,000 watched overseas. More will follow with encore screenings. Since the first such broadcast in 2009 – when Helen Mirren's Phèdre was seen by 50,000 people worldwide – NT Live has achieved a total audience of 1.3 million.

"It's gone much better than anyone expected," says David Sabel, the National's head of digital. "The uptake has been so phenomenal that a broadcast can sometimes break even on UK box office alone." Sabel began his career in the building as an intern in 2008, when he wrote a report estimating that NT Live would need to reach 500 screens worldwide to cover average costs of £200,000. "Now we're on about 650 screens, 260 in the UK." That's a third of Britain's cinemas; Bond film Skyfall screened in about half. What started out losing £30,000 now gets "a modest return".

Sabel was initially "quite sceptical" but was converted, along with the NT's top dogs, when they attended a Met Opera Live broadcast in a cinema: "It felt theatrical. People had dressed up. They were reading programmes." Despite the sense of occasion, no one was sure the format would work for theatre. "Opera is always opera. You've got the music and you can't escape the artform. Our concern was that once you point cameras at a play, you bring the expectations of a film-goer. But we're trying to capture a theatrical event."

So how to translate theatre to the cinema without losing its immediacy? On a screen, even great shows can look hammy, turgid and dead. To avoid this, specialist camera directors are bought in: musicals get music concert directors, Shakespeare goes to drama types. This House falls to Tim van Someren, whose credits also include panel shows and awards ceremonies. "Our aim is to film your performance," he tells the cast at a rehearsal, "not shape your performance for film." He tells me later: "The key is to remember it's not just a play – it's tonight's performance at the Olivier theatre. We don't show cut-aways of audience members laughing, like on TV. But if they're in shot, we don't shy away from showing them."

His first task is to decide how to film each moment. "You can't cut it like EastEnders," he says. "It would be too busy. And someone's face in closeup on a 20ft screen is a bit much." Zoom out to show the whole stage, however, and it's dull. He's got six cameras to play with: two fixed, two tracking, a crane and a Steadicam. "You feel a rhythm. If you're doing a farce, it's very cut-cut-cut. Shakespeare is slower." This House has about 1,000 shots.

He works from two broadcast vans parked outside the National. The cameramen are directed from here; the six feeds, 16 microphones and the soundtrack are mixed together, and subtitles are added – all live. It's a complex procedure, so each broadcast has the luxury of two rehearsals and a private screening to iron out problems. Everything is tweaked for the cameras. The crying of a (fake) baby is turned down for the microphones. Lighting is softened. Projections are brightened. Giuseppe Cannas and his makeup team redo every wig, replacing sturdy theatre weaves with more natural "camera-friendly" hair. Bald patches become a concern. "HD is extremely unforgiving," says Cannas. "Everything needs to be toned down: eyebrows, mouth, contouring, thickness of foundation." For the actors, too, the rehearsals are a chance to get comfortable with the cameras. Lauren O'Neill, now appearing in her second broadcast, says NT Live brings "a new set of nerves. You become aware of how many times you blink. It's very strange." And the cast don't know which camera's broadcasting at any one moment.

So what next? There are no plans to extend NT Live online, but "never say never," says Sabel. Curiously, the broadcasts haven't diminished audiences for regional tours or West End transfers. There will be more collaborations with other theatres, too. Next up is NT Live's first West End show: The Audience with Helen Mirren as the monarch. Tom Hiddleston's Coriolanus at the Donmar follows in January; and Kenneth Branagh's Macbeth, in Manchester next month, has just been confirmed.

"There's been a real shift in our thinking," Sabel explains. "NT Live has built its own audience, who can see eight productions a year without setting foot inside this building. We used to think the ultimate success was someone seeing an NT Live, then coming to the theatre. Now we see it as a success in its own right. Our audience isn't just about the bricks and mortar. It's much, much bigger than that."

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