Like a skilled magician, Hugh Bonneville miraculously produces a hard-boiled egg from his top pocket. As we sit down for lunch in the garden of the Festival Theatre in Chichester, the actor explains why he has just whipped out said egg with a flourish.

“I’m obeying that old advertising slogan, ‘Go to work on an egg’,” grins the actor, as he taps the egg on the table and starts to pick the shell off it. “I’m having a hard-boiled egg for my lunch because if I went for my preferred option – which is a great big wedge of ploughman’s lunch – I’d have to have a nap afterwards, and I can’t do that when I’m doing this play.”

Indeed not. The 52-year-old actor very much needs to have his wits about him as the lead character, Dr Thomas Stockmann, in Henrik Ibsen’s An Enemy of the People, a perennially resonant story about who really pulls the levers of power in society. Dressed in a loose blue shirt and jeans, Bonneville is looking very trim – the boiled egg diet is clearly paying dividends. Vivacious, mischievous, humorous company, he is an Enemy of the Prosaic.

Unsettling modern echoes

The play was written in 1882 but 134 years later, An Enemy of the People still touches on themes that resound with unsettling modern echoes. In this play, which is being performed at Chichester in a version by Christopher Hampton, the idealistic Dr Stockmann is Chief Medical Officer of the Baths.

Much to his horror, he discovers that the popular local spa has become dreadfully polluted. Fortunately, he has the ideal solution: shut down the contaminated baths at once and subject them to a deep cleanse. He is the archetypal principled whistleblower.

However, not everyone in the town views this matter as quite so straightforward. What effect would closure have on tourism, property and commerce? What impact would it have on the town’s reputation? Will the townsfolk pay heed to Stockmann or will they revile him as an enemy of the people? It is a universal story, which has inspired everything from Arthur Miller’s 1950 stage adaptation and a 1978 Steve McQueen film to, perhaps most famously, Jaws. Bonneville laughs that “Stockmann is the Chief Brody of An Enemy of the People”.

Known better for his screen roles, notably the traditionalist Lord Grantham in Downton Abbey and Ian Fletcher, the put-upon BBC Head of Values in W1A, Bonneville says he was drawn to return to the stage for the first time in 12 years because this play clearly still strikes a chord today.

Hugh Bonneville in rehearsals for An Enemy of the People

Principled but unbending

Indeed, you have to look no further than the unfolding scandal of the contaminated water supply in Flint, Michigan, to see how topical this play remains. Meanwhile the actor was enticed by the enduring relevance of Ibsen’s depiction of a whistleblower who is “trying to do the right thing, and popular culture turning against him… each generation finds this theme contemporary”.

In this principled, yet unbending character, Bonneville sees specific parallels with modern day headline-makers. “You look at Edward Snowden or Julian Assange and you think: ‘One man’s hero is another man’s pariah’. How quickly a hero’s status can be flipped. Ibsen charts that rather beautifully and economically.” What also appealed to Bonneville was the fact that Stockmann is “an anti-hero”.

“He’s flawed. He makes perfectly good points, but sometimes you just wish he would temper it and put it across in a better way rather than being self-righteous and smug. He really doesn’t help himself. Sometimes you wish he’d just shut up. Because of his vanity and his naivete, he runs the risk of walking towards the edge of the precipice.”

Downton Abbey may have finished as a TV series at Christmas, but Bonneville is an actor who never seems to put his feet up. He can be seen on screens next week as Humphrey, Duke of Gloucester in The Hollow Crown: The Wars of the Roses, the next tranche of BBC2’s acclaimed adaptation of Shakespeare’s history plays.

Downton on the big screen

Later in the year, Bonneville will also star as Lord Mountbatten in The Viceroy’s House, [Gurinder Chadha]’s film about the Partition of India. After that, he will be “considering my options, as members of the Cabinet say from time to time.”

One project that may well hove into view is Julian Fellowes’ much rumoured movie version of Downton Abbey.

“That’s been whispered about for three years,” he says “The affection for the show is still tangible. I keep hearing that Julian is storylining the movie and is on the verge of writing it. So it will all depend on when he has a script and whether he can herd all the wasps back together – I didn’t want to say sheep! In fact, getting us all together would be more like herding cats or jelly.”

As to the wisdom of doing so, “of course, there is a risk in putting a beloved TV show on the big screen,” he acknowledges.

“Sometimes it works and sometimes it doesn’t. Star Trek worked fantastically, and some people would say that Entourage didn’t. Nor did that seminal work, Holiday on the Buses! But if there’s anyone who knows about the difference between writing for film and writing for TV, it is the Oscar-winning Julian Fellowes.”

He adds: “It would be lovely to revisit the character of Robert for one last turn of the dance floor. Where would he be in ten years’ time? On the Cote D’Azur, wondering how he got there and how long the money is going to last.”

W1A reunion

The other exciting potential reunion on the horizon for Bonneville is with his W1A crew for a new series of John Morton’s coruscating satire about the stifling bureaucracy of BBC management, where all the execs live in terror of what Tony [Hall, the BBC’s Director-General] will think.

The actor, who also starred in the movie Paddington, reckons the comedy has proved so popular because “John is so good at picking up the vacuity of management-speak.”

“Everybody feels that they are Ian Fletcher, surrounded by idiots,” he said. “The only way to get something done is to do it yourself because you risk complete mayhem if you depend on others to get you into a building or to unlock a meeting room.

“By the way, doesn’t the new BBC3 logo look like a product of Siobhan Sharpe’s PR company, Perfect Curve? At least we were doing something right.”

Certainly, Bonneville reckons the W1A story still has some mileage in it. “I’d like to see Ian and his team coming back for one last hurrah. They would come up with the dynamic idea of moving the BBC’s centre in Salford to London and designing a fantastic, half-horse-shoe building somewhere in the west of the capital where they could have studios and actually make programmes in-house and have meetings without electronic entry-pads in rooms not called ‘Steptoe and Son’. As always, John will be very prescient and be way ahead of the Perfect Curve.”

And Fletcher’s natural endpoint? “It would surely only be a matter of time before Ian, who has never made a programme in his life, is running BBC Vision Express, a scaled-down version of the Corporation for the Whittingdale era, I think Tony [Hall] should check his muffins because you never know what’s in them when the pursuit of power is at stake…”

‘An Enemy of the People’ is at Chichester Festival Theatre, West Sussex, until 21 May. Tickets from £10 (01243 781 312; cft.org.uk)