"It's like everyone has their place and their role, so they bring a different dynamic to the various aspects of the piece, a different flavour, if you like. I felt like Rose came in and really evoked something specific, so it was like I had something to grasp on to. 'What is the point of me?' was something I asked myself often."
After Rose's relationship with Afro-American jazz musician Jack Ross last year, this season a more mature character emerges. "She has grown up," says James. "She's calmer and much more settled. Her eyes have opening up to the bigger picture and she finds herself involved in something that really takes shape over the whole series. For once, she is thinking of others and not just herself and. because of that, a lot of good things happen.
"But she will still remain blissfully naive when it comes to many things because that's just her sensibility. She lives so much in the present that she just lets things happen around her.
"I think she just needs to read a newspaper."
However, Rose still cannot resist meddling in the affairs of others and her attempts to kindle romance between the widowed Tom Branson (Allen Leech) and headstrong schoolteacher Sarah Bunting (Daisy Lewis) lead to some awkward moments in the
first episode.
"She has such great intentions with that," says James, laughing. "She has no idea that Sarah is going to be so rude at dinner but it's a happy coincidence that she is because Rose loves a bit of drama.
"She likes it when people are knocked off their pedestals so she enjoys seeing Lord Grantham getting it in the neck a bit.
"But she's also very loyal and if it goes much further, she'll be the first to protect her family. She loves them now so she doesn't want to do anything that brings shame on the family, or indeed would lead to her being sent back to her parents in Scotland."
The new series is set in 1924, and James appreciates the less rigid clothing of the more progressive period. "It's really interesting to see the way in which the world is changing being reflected in the fashion," she says.
"You can see that throughout history and it's amazing. The girls, the women, were cutting their hair off and wearing much more loose-fitting stuff. It was like they were liberated, as they were free to go out dancing and were doing the Charleston. You'd see these amazing dresses that were going up and up so you'd see gradually more calf. It was incredibly risque ... Rose is very much a part of that movement."
But although she is thankful she no longer has to wear a corset, James is not so fond of the equally restrictive chest flatteners some women wore to make themselves seem as manly and strong as their masculine counterparts.
"They're denying their femininity, in a way, just to be seen as a person in this hugely sexist world that they live in," she says.
"They were squashing down their curves and wearing boyish straight lines."
Born and raised in Surrey, the 25-year-old has found getting into Downton Abbey has been a crucial breakthrough, after small parts in films and TV series such as The Secret Diary of a Call Girl, Fast Girls and Wrath of the Titans.
She could soon be joining Jessica Brown Findlay in heading to Hollywood after landing the title role in next year's Kenneth Branagh-directed Disney blockbuster of Cinderella, as well as the coveted part of Elizabeth Bennet in the forthcoming adaptation of Pride and Prejudice and Zombies. But she has no plans to leave Rose behind her, at least for now.
"Making a movie like Cinderella really just feels the same as being on Downton, which is just so incredible," she says.
"The only real difference is the size of the sets and the crew, which were much bigger on Cinderella.
"But everything else was the same. It's funny because Cinderella also has a very interesting cast with great English actors.
"Sophie McShera, who plays Daisy in Downton, was with me as one of the Ugly Sisters, so it felt like a bit of Downton out at Pinewood."
