Debicki portrays the beloved late princess in Netflix's much-lauded period drama.
"In terms of fittings, I would say she's probably spent upwards of 30 hours in the hair and makeup chair while we do stuff," Cate Hall, The Crown's hair and makeup designer, told ET.
Hall said the most challenging moments to recreate were the less public scenes, when her team was tasked with ideating what the Princess of Wales looked like when she spent alone time with her children and family.
"When Diana's just been swimming, and she's laying around with her kids, trying to find that believable Diana within the lesser scene framework" was the most challenging, Hall said.
To do so, Hall and her team spent an extensive amount of time perfecting Debicki's wig, which Hall said is what solidifies Debicki with the recognizable Diana silhouette.
"We probably had at least four, four-hour fittings before we were even close to going, 'OK, this is what we're going to do,'" Hall said. She added that Debicki also dyed her eyebrows for the look, and stuck to a strict schedule of spray tans, nail routines and daily hair and makeup changes.
As for the costumes, wardrobe designers Amy Roberts and Sidonie Roberts echoed Hall's assertion that the private moments can be the most difficult to recreate, but as The Crown's time period moves closer to present day, the costume team was able to source more authentic pieces from the era.
"There's a scene where Diana reads about Camilla and Charles at Camilla's 50th birthday party and she's in a really oversized Adidas T-shirt," Sidonie said as an example. "So there's been a lot of moments where we've been able to source...things that are accessible, good quality, high quality."
Debicki took over the iconic Diana role in The Crown's fifth season, after Emma Corrin played the princess' early years in season 4.
"Obviously Princess Diana was a truly profoundly remarkable, unique woman," Debicki told ET in 2020. "So it's nothing but an honor to be asked to play her."
Debicki's styling in season 5 brimmed with references to real-life Diana's unforgettable fashion sense, including the famous revenge dress.
Amy Roberts and her Associate Costume Designer and Head Buyer Sidonie Roberts told ET the dress wasn't an easy task.
"That 'revenge dress' is such a tricky thing to make," Amy said, explaining that they had to "sculpt it to Elizabeth's body." Sidonie added, "Elizabeth's torso is different to Diana's."
Still, it was important the team nail the look -- Debicki herself chimed in on the dress' importance, telling British Vogue that "when people in my life found out that I was going to play this part, this was the dress that everyone texted me about. And that's when I started to realize how symbolic this dress is to people."
The show's final season now promises similar levels of historic iconography. Late last year, fans spotted Debicki filming in costume as Diana on her 1997 summer trip to the French Riviera, wearing an animal print swimsuit nearly identical to the one worn by the late princess. In the trailer released last month, Debicki also recreates the blue bathing suit paparazzi photo that sold to tabloids for millions of dollars. In first look images of Debicki in costume, fans got a few more glimpses of the princess in her final days before the fateful car crash that killed her and Dodi Fayed.
"I don't really understand how I ended up here," Debicki's Diana muses in the trailer. "Dashing around, and losing sight of myself in the process."
Dominic West will return as Prince Charles in season 6, with Olivia Williams as Camilla Parker-Bowles. Khalid Abdalla is back at Dodi, and Salim Daw will return to play his father, Mohamed Al Fayed. Imelda Staunton plays the queen, Lesley Manville plays Princess Margaret and Jonathan Pryce plays Prince Philip.
The Crown: Part 1 premieres Nov. 16 and Part 2 on Dec. 14 on Netflix.