For leading lady Felicity Jones, working on Rogue One: A Star Wars Story was a complex process. Not only did the 33-year-old English actress have to act like an action hero at work, but she also had to behave like a tight-lipped secret agent when she got home.

Films in the Star Wars franchise are veiled in secrecy and Jones had to ensure that no story details slipped out when she relaxed with her family and friends. "You end up feeling like you are a spy or something," she says.

"You have to be really secretive. You come home from work and your friends and family say, 'Did you have a nice day today?' and you're like 'Yep!'

"They ask if anything exciting happen and you had to be, 'No! Nothing happened.' It was really hard because you are excited about it and you want to talk about it. But the stream of consciousness has to be monitored. I have become excellent at keeping secrets."

The full details of Rogue One might be wrapped in secrecy, but the basic narrative is known. Jones plays Jyn, a young woman who is separated from her father (Star Wars loves a parent-child tale) and raised within the Rebellion, which is battling against the ever- expanding power of the Empire.

"Jyn is thrown into a situation that she is not expecting," says Jones of her character. "She's complicated. That's what I like about her. She's resourceful when she has to be, but she also has had to stand up for herself. She has had to be wilful. She is very determined."

Much like Jones herself, in fact, who has been acting since the age of 12. She came to international prominence after scooping an Academy Award nomination for her performance opposite Eddie Redmayne in 2013's The Theory Of Everything. She might well earn another nomination for her moving performance in the forthcoming family drama A Monster Calls.

"I find when I am playing a character, it is always difficult for my friends and family because Felicity starts to recede and the character starts to take over," she says. "There is always a bit of a crossover and I found that with Jyn. I like to be like her. I hope I am like her. She is pretty cool."

She certainly is. Jyn's story sees her embark on a dangerous mission to steal the plans for the Death Star, the gargantuan planet-busting space station that the Empire will use to further subjugate and enslave the galaxy.

The events in Rogue One foreshadow those that unfold in the very first Star Wars film, 1977's Episode IV: A New Hope, which opens with Darth Vader trying to recover the plans from Princess Leia. Rogue One is directed by sci-fi fan and self-confessed Star Wars geek Gareth Edwards.

"His enthusiasm was great," says Jones of the director. "The whole thing was a slightly dream-like existence because everyone had such affection for it."

The shoot, she notes, was full of "pinch me" moments. "You can't help it, particularly if you are having a hard day and you look at the call sheet and you go, 'Oh, I am going to be working with a creature today.' It is really exciting when some kind of multi-tentacled beast is sitting next to you. It is truly magical."

She recalls talking to co-stars Diego Luna and Riz Ahmed, whose characters form part of the team that she assembles to steal the Death Star plans. "We would be talking seriously, wondering how we were going to play a certain scene and suddenly, we would look up and there'd be a giant monkey hanging out on the set, or there would be a Stormtrooper walking around drinking a latte. It was really amazing."

Another "pinch me" moment arrived when she was asked to deliver the series' most iconic line, "May the Force be with you". She smiles at the memory. "It is one of those great lines. It is like, 'To be or not to be.' You have really got to make sure that you have got it right or you will feel really stupid.

"So I spent ages walking around my house saying it a lot, over and over again, and trying different pauses for different effects." She adds with a laugh: "But then when I actually did say it on camera, I totally just channelled the Force."

She also had to channel her inner warrior. She had to learn to fight and her moves needed to be convincing, especially given the fact that Jyn leads a team with the accomplished warrior characters Chirrut and Baze, brought to life by Chinese actors Donnie Yen and Jiang Wen respectively.

She trained hard, learning the Chinese martial art of wushu. "That side of it was something I was definitely new to," she says of her fight training, "but I actually really loved it and got more and more into it throughout filming and doing stunts.

"Before starting Rogue One, I was working on another film and I met up with a gongfu expert. We met in the evenings and I very gently started to learn wushu. I have to say it takes a lot of practice. It doesn't come automatically. It is hours and hours of getting up early and practising and rehearsing and perfecting it."

She hopes that her work has paid off. Playing the lead in a Star Wars film - which may well feature appearances from both Darth Vader and Leia - is some responsibility. She is aware of the anticipation from fans of all ages.

"The beauty of Star Wars is that it appeals to everyone," she says. "A seven-year-old girl can watch it and a 70-year-old man can watch it, and while they both will get something very different from it, they will still really enjoy it."

Certainly, her family will be keen to see what lay behind all that secrecy. Well, most of her family - she gave her 18-month-old nephew a Jyn action figure and he seemed less than impressed.

"When I gave the action figure to him, he immediately started to chew the head off. I'm not too sure that he was proud of me. He was probably just hungry."