But global fame didn't stop Hugh Bonneville falling foul of the BBC's security system while shooting new comedy W1A at the corporation's London headquarters.
"We were filming in the lobby the other day, 20 of us milling around, and the chaps on the doors were chuckling watching us do it. Then I tried to get back inside to get changed and they wouldn't let me because I didn't have the right pass," he recalls.
"People who have worked here a long time will know what I'm talking about; the bureaucracy of the machine is part of the thing that is so lovable about it.
"And so confusing," the actor adds with a laugh.
London-born Bonneville is reprising his role as bumbling boss Ian Fletcher in W1A, a spin-off from the award-winning Olympics mockumentary Twenty Twelve. Filmed amid the bustle of Broadcasting House, W1A sees the BBC turn its satirical sights on itself.
Ian, former head of deliverance for the London Games, has a new and equally vague job title as the broadcaster's head of values as it gears up for charter renewal and a new licence fee settlement.
As with Twenty Twelve, the corporate jargon is rife. There are hot desks, digital handshake sessions, daily senior team damage limitation meetings, and a 'balancing area' which consists of a huge orange see-saw (a prop, not a permanent fixture, presumably).
On the day I visit the set, the BBC's creative director Alan Yentob is sitting near the aforementioned see-saw, preparing to film a cameo.
Fictional crises discussed - and then discussed some more - in W1A range from Jeremy Paxman falling asleep during an interview with Scottish First Minister Alex Salmond, to claims that Cornwall is under-represented by the BBC.
It seems like a brave move for the corporation to poke fun at itself - indeed, the new show was announced just a day after director of television Danny Cohen hit out at "the daily chorus of BBC-bashing".
Bonneville thinks the organisation is right to show it has a sense of humour.
"It's wonderful. In the same way Sebastian Coe got the joke about Twenty Twelve - that it wasn't actually satirising the notion of the Olympics - and allowed us to film him, we're not having a go at the BBC.
WE are highlighting the strange corporate speak and structures of in any organisation, be it a FTSE 100 or the village hall."
Filming in situ gives the show an authentic feel but it can lead to some confusion.
"We often use long lenses, so the camera will be far away. I had someone come up and start chatting to me, so I had to explain that we were in the middle of a shot."
Among those thwarting Ian's attempts to get anything done are uncompromising head of output Anna Rampton (played to poker-faced perfection by Sarah Parish) and brand consultant Siobhan Sharpe (Jessica Hynes), whose utter ineptitude Twenty Twelve fans will recall with relish.
Bonneville, who lives in West Sussex with wife Lulu and son Felix, admits it can be difficult to stay straight-faced on set.
It's been a busy few months for the star. With filming on W1A wrapped, he's now working on series five of Downton.
And then there's Second World War film The Monuments Men, in which he appears alongside George Clooney, Matt Damon and Cate Blanchett.
"Monuments Men, goodness me, it was all down to Downton Abbey I'm sure... I was fantastically nervous working with these great icons of the screen - some of my heroes, like Bill Murray and John Goodman and Jean Dujardin from The Artist, wonderful people. Of course, as soon as you get on set with them, you realise they are just actors who want to do a good job too," says the 50-year-old.
The corporate sphere of W1A and the aristocratic grandeur of Downton Abbey are worlds apart, but the actor enjoys both.
"The nature of filming is very different - one is faux documentary style, the other couldn't be more sort of classical. I love that variety," he adds.
But surely there are similarities between Ian and Lord Grantham, as both try to maintain order?
Bonneville laughs. "Lord Grantham and Ian Fletcher are both just trying to run a big team and hoping that the wheels don't fall off the vehicle," he says. "They're both men trying to get by."