Kristen Stewart shared how she is preparing to play the role of the late Princess Diana in the upcoming biopic "Spencer," during a recent interview to promote her new holiday movie, "Happiest Season."

The "Twilight" star called the movie a "really meditative project" because it is "a really internalised story" about the princess' life during three crucial days. She told ET that she is trying to read everything she can about Princess Diana, born Lady Diana Frances Spencer, and forget about them afterwards.

The 30-year-old "Charlie's Angels" actress shared that people will find "Spencer" to be really emotional. It will be different from the other stories told about Princess Diana because it only focuses on the last three days before she faces the end of her marriage to Prince Charles.

"There are so many perspectives of her and of her story... it's just not a black and white thing and it's a very slippery, really emotionally-packed story for a lot of people," Stewart explained adding, "This is a really kind of poetic, really internalised imagining of maybe the heaviest three days' time before she does something as gnarly as leaving the royal family."

"Spencer," by "Jackie" director Pablo Larraín, will show Princess Diana still spending Christmas with Queen Elizabeth II at her Sandringham Estate in the 1990s. Her sons, Prince William and Prince Harry would still be young at this time.

Stewart, who was only seven years old when the late Princess of Wales died in Paris in a car crash in August 1997, did not know much about her then. She only remembered the mourners and the flowers placed outside Buckingham Palace following her death.

The "Underwater" star explained that her movie is an "imagining of what that might have felt like rather than giving new information. We kind of don't have a mark to hit, we just also love her," she said.

Stewart described the princess' story as "one of the saddest stories to ever exist." She admitted that she does not want to just portray Princess Diana in "Spencer" but "want to know her implicitly."

Kristen Stewart
Kristen Stewart will play Princess Diana in "Spencer." Andrew Toth/Getty Images