Elizabeth Debicki says closing the role of Princess Diana in The Crown came with a unique weight.

Speaking to PEOPLE before the show’s season 6, part 1 premiere on Thursday,

Debicki says the hardest scenes to film for Netflix’s drama inspired

by the lives of the British royal family were those around the end.

The first installment, told in four episodes, explores the events surrounding Princess Diana’s tragic death in 1997 alongside love interest Dodi Fayed (played by Khalid Abdalla). Part 2 debuts on Dec. 14 with six episodes, focusing on the aftermath of the late Princess of Wales’ death and the future of the monarchy.

“All of the scenes in the lead-up to the end of this season were in a cumulative way, very difficult. We shot them over a number of weeks. I think shooting the sequences where the two characters are being really pursued by the paparazzi was just a really horrible experience. It’s very physical,” Debicki, 33, exclusively tells PEOPLE. “After doing that for a few weeks, we were very drained, very tired. We felt in a way it’s almost like doing a stunt in the sense that your body starts to take on the kind of physical experience and you’re not doing a lot of acting.”

“The feeling was almost like being underwater. You’re just kind in this swamp of noise, of harassment, of being pursued,” she says. “As the actor playing the role, the character is just constantly focusing on where they’re trying to get to, but there’s just this accumulation of obstacles in the way of that, and that was a very kind of difficult, taxing thing to experience.”

PEOPLE previously confirmed that the moment of impact of the car crash that killed Diana and Dodi would be excluded from The Crown. Executive producers Andy Harries and Suzanne Mackie later stressed at the Edinburgh TV Festival that the depiction of the tragedy was approached sensitively on screen.

On Aug. 31, 1997, the Princess of Wales and the Harrods heir were traveling by car in Paris with a driver and bodyguard pursued by paparazzi when their vehicle slammed into a support column of the Pont de l’Alma tunnel, flipped and came to a rest in front of oncoming traffic. Dodi and the driver died instantly while Diana died later in the hospital. She was 36 years old.

The cast of The Crown changed every two seasons to reflect the royals aging through the decades, and Debicki took over from Emma Corrin for the role of Princess Diana in seasons 5 and 6. The star says what shifted for the final season was showing a different side to Diana.

“The script offered different sort of colors in a way of Diana, and I was really grateful for that. I think in season 5, the character is much more isolated. She’s on her own a lot, and she’s going through incredible trials that really test her and test her sense of reality and her sense of herself,” Debicki says of the penultimate installment, which chronicled the breakdown of Diana’s marriage to then-Prince Charles, finalized by divorce.

“When I opened up the scripts to season 6, what I saw Peter [Morgan] had created for me to follow, was this sense of someone who was freer, who was kind of blossoming into a different version, who had been through the fires and had sort of come through stronger and more focused,” she adds of what the showrunner imagined.

Part 1 picks up with Princess Diana’s holiday in Saint-Tropez with her young sons, Prince William (Rufus Kampa) and Prince Harry (Fflyn Edwards), hosted aboard Dodi’s family’s yacht.

“I think in the beginning of season 6, Diana just wants to have a good vacation with her kids. She really just wants to have a nice time, which is a difficult thing to do in life, to say, ‘Everything goes to the side. We just have fun,’ ” Debicki says.

“She really has this intention to have fun. And then of course, the way that Peter’s written it, this relationship with Dodi Fayed just tumbles together and they find each other and they’re surprised at what they find together in terms of the chemistry,” the actress continues. “All of those pieces of making this part of the story were just a joy to do, to experience.”

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On what she most hoped to bring to the final season, Debicki says it was all about Diana as a mother.

“I felt like I really had an opportunity in this season to just really focus on the relationship between Diana and her boys. Just that love, the joy, and the relief of spending time with them, and feeling really truly yourself when you have your children with you, and a real sense of kind of purpose,” Debicki tells PEOPLE. “That was so important for me because obviously we know where the story is heading. I really focused, whenever there was an opportunity, to show how incredibly alive and beautiful this person was.”