Naomi Watts' personal trainer has revealed the gruelling three-month regime that transformed the actress into Princess Diana.
Fitness guru Simone De La Rue told Health & Fitness magazine the 44-year-old mother-of-two worked out for an hour a day, at least five times a week to get into shape for Diana.
Simone, who is Australian born and New York based, is famous for her dance-based workouts which are particularly popular with celebrities because they "are a fun way to get your sweat on and burn calories".
"When working towards a specific goal with a client, I encourage them to train five or more times a week, for one hour a day," Simone said.
"I set a programme that constantly challenges my clients and, during the last three weeks, we really increase the intensity."
Naomi plays Diana in the film of the same name which focuses on the last two years of her life and her love affair with heart surgeon Dr Hasnat Khan.
The film will be released in Australian cinemas on October 10.