"Everything [Diana wore] had a reason behind it – she was quite methodical in the way that she dressed," he told Vantage magazine.
"She took the colour into consideration, the style and the event."
The film, Diana, which stars British-Australian actress Naomi Watts, documents the last two years of her life and Jacques was tasked with producing replicas of some of her most iconic eveningwear.
"My initial reaction was 'great, exciting' because I'd always like Naomi Watts and I think she's a great actress," he said.
He spoke about the infamous ice blue shift dress that caused a media frenzy when Diana wore it to the ballet in 1997.
"She actually wanted to go shorter!" he laughed, revealing that it was actually Diana's butler Paul Burrell who advised her against it.
"We took it as short as we could and she respected the fact that obviously she was still a princess and there were certain restrictions with length and with how much cleavage she was showing."
He incorporated black into the film costumes – a colour she particularly enjoyed wearing as it had once been denied to her. "You can only wear black for mourning in the Royal family, so the first thing she did when she was pulled away from the monarchy was to wear a black dress," he explained.
Jacques was introduced to Diana at the London Designer Collection by then editor of British Vogue, Anna Harvey, and became the person responsible for steering her towards a more glamorous and sophisticated look, “taking her away from the frills”.