King Kong actress Naomi Watts was so determined to portray Diana, Princess of Wales accurately that she stayed in character even off camera, her co-star has revealed.
Canadian star Cas Anvar plays Diana's boyfriend Dodi Fayed in the movie about the last two years of her life and he has revealed that making the film was an intense experience.
"I had heard about Naomi before, that she was part British and part Australian, and I was expecting a little bit of banter but she was so in character even when we weren't shooting.
"She was so poised and gracious and in control in the same way Diana was always in control. That was actually a very powerful energy as the other actor and it helped me interact with her."
Speaking at the Los Angeles premeire of British movie I Give It A Year, Cas said Naomi took the plunge when it came to embodying such a famous person.
"It was quite a commitment she took just by taking the role because how do you portray an icon? But she dove headfirst into it."
He added: "Every day I would see the paparazzi shots and it was the first time that level of paparazzi had been around while I was filming. It was a little overwhelming and that is when I realised how much of an impact the film would have. We were not just doing a story, but a story about the most popular celebrity int he world. Everybody knows who she is, was in love with her and heartbroken when she died and now they are hungry to see this film.
"It was thrilling and scary and overwhelming all at the same time."
Diana is out in UK cinemas next month.