The 44-year-old British-born actress said that she wanted to channel the late royal, rather than mimic her.
“I don’t want to get caught up in mimicry - that’s the worst thing,” she said.
“I was to try to embody her and get the essence of her. Those things are really important to me,” she added.
The film, out next year, follows Diana’s love affair with Pakistani heart surgeon Dr Hasnat Khan, played by Lost star Naveen Andrews.