Timothée Chalamet stars as Willy Wonka in the film, in theaters Dec. 15
At the Wonka world premiere Tuesday held at The Royal Festival Hall in London, Chalamet, 27 — who wore a colorful Tom Ford suit and Cartier necklace — posed with costars on the red carpet and took selfies with fans.
Though Jenner didn't pop up on the red carpet, a source tells PEOPLE the Kardashians star, 26, did attend the U.K. premiere and its afterparty.
Other cast members in attendance included Sally Hawkins, Rowan Atkinson, Hugh Grant, Olivia Colman, Keegan-Michael Key and Calah Lane.
In the new film, Chalamet plays the titular character, previously portrayed on the big screen by Gene Wilder in 1971 and Johnny Depp in 2005. The Oscar nominee told PEOPLE at the London premiere that the "challenge" of the role was to "live and love the tone of it."
"I realized everything I've worked on [has had a] grounded seriousness to it, and here I had to loosen up and play, kind of like I did in high school, but I hadn't done in 10 years or something," he said. "That was the learning curve. But once it was in full swing, then it felt like we were cookin.' "
Jenner also recently showed up to watch Chalamet host Saturday Night Live in New York City earlier this month.
The pair have been spotted together in recent months, including at a Beyoncé concert in September, where they kissed in the crowd during the show.
"He makes Kylie happy. He has his own life and understands that Kylie needs to prioritize her kids," said the source. "He is charming, very loving and protective of Kylie. She likes that he is a private guy."
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Last month, in an interview with WSJ. Magazine, in which she revealed her new clothing line Khy, Jenner said she's a fan of shows like Game of Thrones and listed Chalamet's film Dune as one of her recent favorites. When asked if she's a fan of Dune, the 2021 movie that also stars Zendaya, Jenner said with a smile, "I do love that movie."
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