It's a weird thing, says Saoirse Ronan, that the first movie in which she used her Irish accent was set in a fictional Eastern European country. But that's the world of Wes Anderson for you. Expect the unexpected, allow the familiar to emerge in an unpredictable way.

The Grand Budapest Hotel, the new feature from Anderson (The Royal Tenenbaums, Moonrise Kingdom), is a rich, funny and poignant comedy-adventure, set mostly in the 1930s, in the imaginary Republic of Zubrowka. Ronan plays a resourceful young woman, Agatha, who works in a bakery, becomes engaged to a young hotel employee and plays a crucial role in the righting of wrongs and the rescue of a treasured possession.

Ronan remembers hearing through her agent, she says, while she was filming in Louisiana that Anderson wanted to talk to her about a part. ''I thought he was joking. I couldn't believe it. But then I got an email from Wes, saying 'Dear Saoirse, if you don't mind, would you read my script?'

''Of course I read it, and of course I loved it. Then we met in London a few weeks later and had dinner.''

Before they sat down for a meal, they ran through a few scenes, talked about accents, about what Agatha would sound like. ''It's usually the first thing I ask about. And he wasn't sure. We tried soft Germanic, we tried English, we tried American, and then he said: 'You know, why don't you just do your own Irish accent?'''

She had to pull back a little bit, she says, laughing. ''I had to make it a little lighter, for people to understand; mine really is quite strong.''

The film stars Ralph Fiennes as the domineering, charismatic Monsieur Gustave, concierge of The Grand Budapest Hotel; newcomer Tony Revolori plays his young protege, Zero, Agatha's fiance. There are many cameos from Anderson regulars such as Bill Murray, Owen Wilson, Adrien Brody and Jason Schwartzman, but she did not have scenes with them.

''I pretty much only worked with Tony and Ralph, and had a little bit with Adrien, and we had one group scene together,'' Ronan says. But she felt very much part of the team, because of the way Anderson approaches his work.

He doesn't like to move from one place to another when he's shooting; he prefers to stay in one spot and find all his locations nearby; and he likes to keep cast and crew together in one place. For the film, he found a town called Goerlitz, on the border of Poland and Germany, and it gave him what he needed, and cast and crew stayed in the same hotel.

''It was this lovely little cross between a B&B and a hotel that looked as if it had been designed by Wes,'' Ronan says.

On Fridays, everyone went to a restaurant called St Jonathan's, across the road from the hotel, she says. There was an immediate sense of camaraderie. ''You didn't feel like you were stepping into Wes' world in the morning when you went on set, you felt like you were part of it from the get-go.''

Ronan, who turns 20 this month, is a seasoned performer, whose roles include the young Celia in Atonement, for which she received an Oscar nomination; the haunting figure of the murdered girl at the centre of The Lovely Bones; a teen assassin in Hanna.

Agatha is a relatively small role and in some ways quite straightforward. Yet Ronan was a little nervous, she says, about being a new member of Anderson's cast when so many other actors were part of his informal repertory company. ''It was all there for me. Those characters are ready to go as soon as you get there. That's what's quite lovely about it all. It's fun and theatrical and very physical as well. Every movement was planned and decided upon before we did it. It was all part of Wes's master plan.

''His attention to detail is so great, he's so specific and so meticulous, in an amazing way. He's a real artist, and you feel like part of a painting.''

It helps, she adds, that he likes to use real locations or specially built sets, and that special effects are a rarity. Ronan has a couple of new projects, including How To Catch a Monster, the directing debut for Ryan Gosling, who also wrote the script. The cast includes Christina Hendricks, Eva Mendes and Matt Smith. ''We all became incredibly close. Again, we all stayed together, and in some ways it was very similar to Grand Budapest, and in other ways it was completely the opposite – the way Ryan works, compared to Wes.''

She's now filming Brooklyn, Nick Hornby's adaptation of Colm Toibin's novel of the same name, the story of a young girl who comes to New York in the 1950s. She can use her own accent, she says, but this time she won't be in a fictional Eastern European republic – she'll be playing an Irish girl in an Irish story.