Did you prepare for the role by reading Sir Christopher Meyer's book 'DC Confidential'? Or are you not that sort of ambassador?

Dan and Lizzie spent time in Washington with Sir David Manning (the current ambassador) and his staff and were not only fonts of wisdom but also gave me the transcripts of all their interviews. The BBC made their documentary archive available, which had some classic diplomatic and fashion disasters to emulate and avoid, and I read Sir Christopher's book. Mostly, though, since we were creating an ambassador who was enough of a maverick to have ruffled feathers all along, I locked myself in a room with Dan and Lizzie and we played 'what if'. What if the ambassador wasn't the public school Oxbridge graduate we expect? What if he was a close friend of the PM? What if, when our story begins, he was about to enter politics proper? On a more superficial note, I got the braces idea from Sir Christopher's habit of wearing red socks all the time and Michael Grade's famous braces. Only halfway through shooting did I find out that everyone hated them – but by then it was too late!

	Ruffling feathers: 'The challenge was to create a flawed character who struggles to do the right thing'

You play a British ambassador striving to save the world from terrorist attack. Isn't that a bit unrealistic? Apart from everything else, Blair emasculated the Foreign Office years ago …

An ambassador, in the words of Henry Wooton, 'is an honest man sent abroad to lie for his country'. His job, traditionally, is simple: to prevent war. It might be argued that that has changed over the past five years, given the recent doctrine of the pre-emptive strike, but mostly they are there to quell fears, calm misunderstandings and whisper in ears. Reading Sir Christopher's memoirs of his time in Washington during the decision to go to war with Iraq was a real eye-opener into just how influential these unelected representatives are: whom they know, who takes their call – who trusts them can have a massive effect on international relations. On top of all that, Mark Brydon's a maverick anyway: everyone – literally all the parties, including the oppressed population – stands to gain from the invasion and regime change that the world hurtles towards in the story, but Mark smells a rat and he's reached a stage in his life and career when one more cover-up might be one too many. What's unusual in the script – and for me as an actor – is that he has to solve these problems with diplomacy, manipulation, guile and not a gun, a bag of money and a horse.

Isn't your character too dashing to have gone through the hard grind of the Civil Service?

You've obviously never spent any time with diplomatic high-fliers. These men and women have to be fabulously charming, utterly adaptable and, for good measure, completely ruthless. Their entire career rests on their ability to prise pieces of information from their mute counterparts while giving nothing away themselves. Apart from which, a good number of them are actual spies, and you don't get more dashing than that.

What did you and the scriptwriters do in August when life started imitating art during the British airport security alert?

I don't think there was ever any doubt that The State Within deals with things that are and would be in the news. That's the territory of our factual life and where better to explore our responses than through fiction? Although our story starts with an explosion set off by a British Muslim and the ramifications in Washington, it quickly becomes a conspiracy thriller set on the world stage. As people die around me and there's a rush to war, it's down to the ambassador to unravel the truth. Much the same could be said for all of us: if the show contains any message at all it's to examine what we're told carefully, work out what must, self-evidently, be true and who has what to gain from what. Hopefully, and in an entertaining way, the story brings us face-to-face with the impossible job of reconciling the need to tell the truth to the electorate, protect our commercial and strategic assets and do the right thing. Something's always got to give. The Prime Minister got it right recently … I wouldn't want his job.