Well, of course, Hugh Bonneville would be cast in Paddington - this Christmas's big-budget CGI spectacular, which brings one of the most cherished children's characters in the canon to the contemporary multiplexes. He seems the very spirit of Paddington himself - slightly crumpled looking, cosily plummy without being alienatingly posh.
And he is already a well-established international ambassador for all things romantically British, thanks to Downton Abbey. Oh, and he's a real, born-and bred Londoner, so well qualified to tell the most London-centric of stories since Mary Poppins. "Paddington was a very vivid part of my childhood," he says. "Mum and Dad used to read me the stories . . . And then when I was older and was able to read, these were the first stories that I read myself. I felt a genuine connection with Paddington, he was my pal, my thing, my buddy, and I wanted to go up to central London and eat marmalade with him."
In Paddington he plays Mr Brown, a risk analyst and fusspot father who becomes most anxious when his family insist on inviting a small, accident-prone bear from darkest Peru, whom they have stumbled upon in Paddington station, to take up residence in his elegant, beautiful family home.
On screen he seems entirely comfortable acting against the bear, which must be challenging considering it was added in afterwards with computer technology. But they had voice actors and a physical comedy expert and even, "sometimes we had a stick with a bit of sticky tape on it which was the right height for his eyes. Sometimes we had nothing at all."
There were, he says "so many different forms of Paddington around us that he was surprisingly real. All acting is about using your imagination in some way, shape or form, suspending your disbelief and convincing an audience that something that you have learned is being done spontaneously. It was just a case of projecting your imagination a few feet in front of you. As far as I'm concerned," he says.
Does he, as a parent himself (his son Felix is 12 years old) relate to the parental anxieties expressed by Mr Brown? He responds by first quoting the famous Phillip Larkin poem, "They f**k you up your mum and dad," at length (he once played Larkin on telly) before adding, "I think every parent can identify with those instincts. It's just that we've characterised them in quite a bold way," he says. "One wants to be protective of one's offspring, but also you need to let them fly. And Mr Brown has gone down the path of being over-protective and rather stifling his family, even though he doesn't realise it, like all parents do he thinks he is doing the best for his children. And actually it's the arrival of Paddington that allows him to reassess where he's at."
The film is a love letter to modern London, a tender and cosy look at the city, but, of course, the process behind the picture-postcard view was more complicated.
"You're up against it when you are filming in real streets. The people of Primrose Hill - they were very tolerant with us. But having a film crew bashing around your street for hours on end and days on end can be quite frustrating," Bonneville says. Today he is polite and smiley, and ever so slightly brisk - hinting perhaps at a touch of weariness from having been at the centre of a big-budget film junket for some time.
Though not a leading man, he has recently become a big name star. Mainly thanks to Downton really, and to his alter-ego there, Robert Crawley, the Earl of Grantham, a role which has made him famous across the world, and parlayed him into high-profile feature films. He was a successful actor before, to be sure, with an impressive career in theatre and television, knocking around on British television in shows like Midsomer Murders and The Vicar of Dibley, as well as having a memorable turn in Notting Hill alongside Hugh Grant, but the last few years have undoubtedly launched him into a new league.
It's Downton which no doubt brought him to the attention of, for example, George Clooney and the casting directors of Monuments Men, in which he starred last year. "It's interesting how it's still growing around the world," he says of the show's mammoth, and unexpected, success. "I had my first letter from China the other day. Someone saying something along the lines of what a fascinating depiction of Chinese society it was, which had never occurred to me. But the fact that it's resonated in every country - it hasn't flopped as far as I know in any country that it's been shown in. I'm not quite sure why that is, but I think probably it's because there are a variety of characters that engage you. People invest in these characters. They get very very possessive of these characters and get furious if one of them dies, or if so-and-so does something which they think is out of character. Which is lovely. Peculiar, but lovely."
He was in the National Youth Theatre growing up, and went on to read theology at Cambridge. He then studied acting before launching his career in theatre, with a two-and-a-half-year stint at the Royal Shakespeare company, and appearances at The National Theatre. "That's very much my training ground, my stomping ground," he says. "But I haven't done theatre for years now, so that's going to have to change."
The problem is finding the space in his schedule these days. "A lot of it's been timing," he admits. "What with doing Downton six months of the year, and then (the sitcom) W1A, and then opportunities like Paddington, there hasn't been much time available. I'm offered quite a few plays that involve white tie and tails and I don't really want to do that now. I want to give up my stiff collar next time I'm on stage."
Bonneville grew up in Blackheath, in South-East London, the youngest child of three. His father was a surgeon and his mother a nurse. But though of a scientific persuasion, his parents cultivated an interest in the arts in their children. "Mum and dad were great theatre-goers, and used to take me a lot to Greenwich theatre. I remember seeing a famous production of A Midsummer Night's Dream by Peter Brook when I was about seven or eight, and having the life frightened out of me seeing Bernard Miles playing Long John Silver at the Mermaid. Being in a darkened room being moved by stories in a shared experience with other members of an audience was something that was very much part of my childhood. My brother and sister who were older than me were terrific actors in school, I used to love going to see them in plays, so I got the bug from there really."
These days he lives in West Sussex with his wife and young son, and despite a pretty hectic schedule, manages to get a reasonably good balance between home life and working life. "Downton shoots about an hour from my home in either direction," he says. "The castle is an hour one way and the studio is an hour the other, so at least I'm not filming in Reykjavik."
'Paddington' is in cinemas now.