Film: Lily James talks about her new film, the Guernsey Literary And Potato Peel Society

LILY James looks so exquisite in period dress that it is almost a surprise when you find her in modern clothes. It really started in Downton Abbey, with those big sublime hats, and continued through Cinderella, War And Peace, Darkest Hour and even Pride And Prejudice And Zombies, and now her latest film, The Guernsey Literary And Potato Peel Pie Society.

Of course she has played modern roles, like those in Baby Driver, Burnt and Fast Girls, but her face is so lovely she looks like she belongs in a different time. But here she is, a perfectly 21st-century woman, in an off-the-shoulder white shirt and delicate gold jewellery.

"Growing up, I always loved the romance of period dramas and the scale of those stories and the escapism of being in a different time," she says. "I think that the storytelling is so rich and I used to love Kate Winslet and Titanic and all those type of films.I had always wanted to do this, so it's been really amazing for me to get the opportunity to work in so many period dramas - and I'm very much looking forward to not now as well!"

That wish will soon be granted, when she appears in Mamma Mia 2 as the younger version of Meryl Streep's character Donna, and in Little Woods, opposite Tessa Thompson. She reflects: "It was a great year. Starting out with shooting this film, which was honestly one of the most wonderful experiences, and then finishing the year with Mamma Mia was great."

First comes the lengthily-named Guernsey, an adaptation of the beloved best-selling book, about young author Juliet Ashton, played by James, who is living in post-war London when she receives an unexpected letter from a farmer on the island. She eventually travels there, hoping to write about the unusual book club he has told her about, which was formed by his neighbours during the Nazi occupation in the Second World War.

"These characters get under your skin," she enthuses. "They are so unique and brilliant and then when I read the book I challenge anyone not to fall in love with the people and the setting and the whole world that is created, I just couldn't stop thinking about the different people on this island."

The film also provided an unexpected history lesson for James, 29, who learned for the first time about the occupation of the Channel Islands and how people were locked on them for years. "I had no idea they were occupied, I mean it's insane," she says. "The Germans were locked there too along with all these people. There was no contact, they were completely stuck, having to live together, with your enemy. I had no idea and it is fascinating the more you read about it.

"All of what my character Juliet discovers being there and going through all the old newspapers and the library and finding out all the stories, I found it very illuminating."

But unfortunately for James, the film did not provide an opportunity to travel to Guernsey, because of the difficulties of making the very modern island look period, and because of the challenges of transporting filming equipment to the location by either boat or plane.

"We didn't end up shooting there," she says. "I think the producers all really tried when they were making the film, along with everyone on Guernsey. They were really keen to make it work but in the end because it has to be period, it has to be set at a certain time, it didn't work out logistically so we went to Cornwall to this beautiful place where they chose parts of it that really reflect and look like Guernsey, but it was very sad not be to be there to shoot it."

Some parts of the film were made on the island, she adds: "They did shoot the big exteriors and the look-out towers, which the Germans built, though. That was all Guernsey."

However, the film did allow the opportunity for an unexpected reunion of Downton Abbey cast members. Joining James in the cast is Jessica Brown Findlay (who played Lady Sybil) and Penelope Wilton (who played Isobel Crawley). And for Brown Findlay, who plays founding member of the literary society Elizabeth McKenna, who is mysteriously missing by the time Juliet arrives on the island, it was all a delightful surprise.

"I had no idea until we had a read-through!" she grins. "I obviously didn't know everyone that was going to be involved and I turned up and I was like, 'Hello, hello, hello'. Obviously I never got to work with Lily because Lady Sybil had passed before she came to visit Downton, but that was really wonderful because we knew each other and it was very exciting."

She was particularly overjoyed to reunite with Wilton, who she had already shared scenes with in the ITV costume drama.

"It's really wonderful to go back and work with an actor you have worked with before and Penelope is majestic. When I did Downton, I was really a massive fan of doing scenes with her. She was always a very generous performer and it was really great to go back. It just means you're very trusting."

Like James, Brown Findlay, 28, is also no stranger to the costume drama after her stint with the Crawley family in Downton, but she says the part of Elizabeth was too good to turn down.

"There is always an element of you don't want to repeat yourself or tell the same story. It's always about story really and anything that is not right now can be described as period or at least not relevant, that can be the assumption. But that never seems to be the case when I read scripts and a story like this really resonates. Just because no-one is using an iPhone doesn't mean that it can't be challenging.

"Elizabeth can be so frustratingly bold. You kind of want to shake her sometimes, but she's herself to a fault and those character are really quite rare to come across. She's not looking to be saved at all, she's out to do the saving and I think that is quite rare. She's not perfect and I love that."

• The Guernsey Literary And Potato Peel Pie Society is released in UK cinemas on April 20.