The film Goodbye Christopher Robin is a biopic about Winnie the Pooh author A.A. Milne and his wife Daphne, played by Domhnall Gleeson and Margot Robbie. And for both actors, playing these upper crust English characters was a challenge, especially as the story dips into some very dark emotions.

Margot Robbie And Domhnall Gleeson star in 'Goodbye Christopher Robin'Margot Robbie And Domhnall Gleeson star in 'Goodbye Christopher Robin'

Robbie was drawn to the project because she grew up with the books. "One of the reasons I wanted to do this project was because my mum used to do voices for my Winnie the Pooh and my teddies," she says, "which is what Daphne does to Christopher Robin in this film."

She says that understanding Daphne was the key. "Daphne definitely wanted to have a girl," Robbie says, "so I think that made it difficult for her to connect with Christopher Robin. She's a very complicated woman, and I didn't want to shy away from any of her faults. But you also see how instrumental she was to the creation of Winnie the Pooh. She's the driving force, without realising that it was putting a huge strain on her family."

Watch the trailer for 'Goodbye Christopher Robin' here:

And then Robbie got deeper into the character. "I was a little hesitant," she admits. "I haven't played a character like Daphne before, and I always get imposter syndrome anyway! Playing someone so posh and English and so removed from myself, I really have nothing in common with her. But it's the stuff that scares you the most that are the funnest choices to make."

Her costar Gleeson agrees. "I think Margot makes amazing choices," he says, "taking on the things that seem like a real challenge, so overcoming it makes it rewarding."

The actor certainly had his challenges playing the author. "It was all the stuff about PTSD and the father-son thing," Gleeson says. "Those are new things for me to tackle. I loved the family dynamic, coming out of World War I and all the trauma, and all the beauty in the small moments, and being rescued by those. That is what matters in this film. And the fact that Winnie the Pooh came out of it is the big bonus on top!"

More: Read our review of 'Goodbye Christopher Robin'

Gleeson was also particularly fascinated by the way the film explores the effect of fame on this little boy whose personal life is suddenly being read around the world. But he wants to assure viewers that the movie doesn't give the books a dark undercurrent. "Winnie the Pooh makes so many people happy," he says. "This film will not ruin your childhood! I would like that on the record."