"I think that they were really young on the first one and now, here we are," Han told Insider of the franchise leads while discussing their evolution from the first movie to the third and final installment released on Friday.
Condor and Centineo filmed the first "TATB" movie in 2017, portraying love interests Lara Jean Song Covey and Peter Kavinsky, respectively.
The film was released the following year and became an instant success for Netflix. Condor and Centineo became rom-com royalty, gained millions of followers on social media, and landed more roles in Hollywood.
In addition to penning the novels that inspired the hit film trilogy, Han has worked closely with the Condor, Centineo, and the crew as an executive producer on all three films.
"They've both grown so much," Han said of the two stars.
"I think they've really been tested in a way, by having this huge success and then coming back and doing a great job and being, I think, really humble and excited to be there," she added. "We couldn't have asked for more from both of them."
Han also said that she's enjoyed seeing Condor and Centineo's effortless chemistry on-screen, whether it's in scripted scenes or ad-libbed moments.
The author said that one such scene occurred in "TATB 3," when Lara Jean and Peter visited Little Cupcake Bakeshop in NYC during a school trip.
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There, Lara Jean revealed that she didn't get accepted to Stanford University and their plan of staying together was in jeopardy because they wouldn't be attending the same college in the fall.
"I love the way it's shot," Han said. "I love the way they interact with each other. "It has some really natural moments in it that weren't scripted, that I love."
Han said that an improvised moment took place near the end of the scene, after Peter suggested that Lara Jean transfer to Stanford after her freshman year and be with him.
Feeling assured that they could make long-distance work temporarily, they held hand across the table and laughed.
Then Peter leaned into Lara Jean and said, "You was tripping out. Why?"
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In response, she said, "I was scared."
"I think when we were pulling out of the scene and they were sort of relieved, there's a line there that felt very natural to me that I don't think was in the script," Han said.
The writer also said that another highlight from "TATB 3" took place in the car after Lara Jean and Peter's bowling date.
Peter expressed being skeptical of his estranged dad's sudden interest in wanting to be part of his life again. He said that there was "nothing worse than not feeling chosen" by his father, who remarried and had two sons following his divorce from Peter's mom.
In response, Lara Jean kissed his hand that was intertwined with hers and said, "I choose you, Peter Kavinsky." He said the same thing about her, and they hugged.
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"I think my favorites are always when it's Lana and Noah showcasing their amazing chemistry with each other," Han said. "They're just both so compelling to watch on-screen together."
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