Film: Margot Robbie and Domhnall Glesson talk about their roles in Goodbye Christopher Robin

He may have been a bear of very little brain but Winnie The Pooh is a global superstar. His image adorns clothes, lunch boxes, pencil cases and furniture, but the story behind the beloved bear and his friends Eeyore, Tigger, Piglet, Kanga and Roo is more sad than most readers know.

The characters were the childhood toys of author AA Milne's own son Christopher Robin, who was known by his family as Billy Moon but lent his given name to the boy in the books.

A new movie about the Milne family and the invention of the world of the Hundred Acre Wood details how distressing young Billy found it when his favourite friends became public property and the woods he played in with his father became famous around the globe.

Goodbye Christopher Robin, starring newcomer Will Tilston as the little boy, shows how post-War Britain seized on the happy tales of a boy and his animal pals and he was placed in the glare of the spotlight as he was trotted out to teas and signings and parties as a result.

Star Wars actor Domhnall Gleeson plays the part of Milne in the film, while Margot Robbie ditches her Australian accent to play his wife Daphne, who both became wealthy and famous beyond their wildest dreams because of Pooh.

Sitting together in a London hotel room, the pair said learning how deeply the stories affected the family changed the way they saw the books.

"I think the world that the film creates, what they end up setting up for themselves as a family, is a beautiful place to go," he says. "That is what I think the books were for everybody in post-war England and then the world, it was setting up a safe place where you can really connect with other people. I think that is what the film gets to, but in real life it's harder to get there than just opening a book, you are going to have to fight through some stuff, so going on that journey with Will and with Margot and everybody, that was part of the joy of it. It was getting to the moments of beauty."

Robbie is also keen to defend Daphne, who was the one who brought her son the stuffed toys to play with but at times appears cold and distant.

"I did like her, I like all the characters I play, whether they seem likeable to the audience or not," she enthuses. "When you play them, I think you start seeing everything from their point of view and it's hard not to understand why they are doing things, so to me she made total sense, but on first read I could see that there was (something).

"Looking at the way she behaves as a mother from a modern-day perspective is difficult, but once you do a bit of research and understand what parenting was like back in those days, you realised it wasn't that abnormal to see your child so infrequently.

"I also did see a lot women in my family who have similar character traits to Daphne, definitely the pragmatic side of her, the 'don't cry, get on with it' sort of thing.

"I kind of had seen that before so I knew that those sort of things could come from a very loving place, even if they didn't sound loving when they came out of her mouth, without a filter."

She adds: "She does it all with the best of intentions, never realising the strain it will put on her family. They never expected the Winnie The Pooh stories to become the phenomenon that it did and they definitely didn't expect their son to become one of the most recognisable faces in England."

Robbie herself grew up with Winnie The Pooh, saying: "One of my favourite stuff toys was my Winnie The Pooh bear so he was omnipresent throughout my childhood." But for Irish actor Gleeson, the son of Harry Potter star Brendan, growing up in Dublin meant a very different selection of children's books.

"The only Pooh in my life was in my nappy," he jokes. "I didn't come to Winnie The Pooh until much later, until I was 33, whenever we were shooting this film.

"That is when I did the prep for it and read them because I had Irish-language books and stuff when I was a kid, I didn't have Winnie The Pooh."

His childhood without the characters also meant the absence of the Pooh sticks game, but he has since more than made up for it. "I didn't know it so didn't play, but in the movie we got to play on the actual bridge where they invented it in real life."

While Robbie, 27, had the advantage of previous knowledge of the material, she did have the challenge of the cut-glass accent of her character, and took lessons from a classic film source. "Brief Encounter!" she exclaims. "It was one of the first things I was told to watch to understand a 1930s aristocratic accent and I loved it. I've never done an accent like that before and that was really great. Joan Washington was our dialect coach on set who herself is, to me, the poshest lady I've ever met too, so that was helpful for Daphne too.

"She was the first person to say to me 'if you're upper class, you don't say 'pardon', you say 'what' because you're not apologetic of anything. I was like 'wow', I thought as Daphne I should say pardon but I clearly don't understand this world at all, Joan was my way in."

• Goodbye Christopher Robin is released in UK cinemas on September 29.