According to News Ltd., she reportedly worked with a dialogue coach for six weeks, and was also outfitted with a prosthetic nose, four wigs and several replicas of Diana's gowns (including a few of the originals) to help her better resemble the late Royal.
"Theres one particular Azagury dress that was completely unaltered," Watts told Vanity Fair. "I was surprised how short she wore it, quite risque! There's been a few eerie moments and that was one of them."
This was not the only time that the actress felt Diana's presence. Watts also explained that she has had dreams about Diana and felt as though they were spending time together.
Other than her physical appearance, Watts is also committed to giving Diana her due, and is concerned about the reactions of the Princess's children, Princes William and Harry.
"We're trying to show her in the best possible light as a human being, though, full of different layers. It's a story that should be told," she said. "But it's tricky because they are here, the boys. I think about that a lot."
"Diana" hits theaters in 2013.
[via Vanity Fair and News Ltd]