'Little Women' 's Saoirse Ronan, Timothée Chalamet Are 'a Bonfire' of Chemistry, Says Greta Gerwig

If there is one thing that Greta Gerwig can guarantee about her upcoming film adaptation of Little Women, it’s that watching Saoirse Ronan and Timothée Chalamet play beloved literary characters Jo March and Laurie will break some hearts.

Gerwig says that Ronan, 25, and Chalamet, 23, whom she directed in 2017’s Oscar-nominated Lady Bird, have a palpable chemistry that is almost combustible.

“There is some true pairing between them that feels like what’s in the tradition of great cinematic pairings,” the writer/director, 36, tells PEOPLE and EW. “They have an energy between them that is like they become a bonfire when they’re together. They’re both so alive and they’re both so talented and so smart and so young. When you put them together it’s like combustion.”

And, according to Gerwig, they are the perfect actors to bring the proper fire and emotion to Louisa May Alcott’s beloved heroine and her suitor.

“It breaks my heart, every time,” she says of watching Ronan and Chalamet in a particularly emotional sequence between the two characters. “They are both wonderful dancers. I think of them as my children, I just adore them. They are spectacular actors. I don’t know how they do it, I mean it’s magic. I direct them, but it’s all there.”

Gerwig also has high praise for Emma Watson, who plays the eldest March sister, Meg.

“To me she embodies everything that I was interested in, in terms of who the March women were,” Gerwig says. “She is lovely, she is way out there trying to do everything she can [with her activism]. And I think, in terms of what she did with the character of Meg, she has so much open-heartedness and so much love combined with that much intelligence, it’s heartbreaking and potent. Because she’s absolutely herself with understanding the struggle of who that character is.”

Based on Alcott’s beloved coming-of-age novel about the March sisters, the film stars Ronan, Watson and rising stars Eliza Scanlen (Sharp Objects) and Florence Pugh (Midsommar) as Jo, Meg, Beth and Amy March; Chalamet plays Jo’s love interest Laurie, Laura Dern is the March matriarch Marnee and Meryl Streep is sharp tongued Aunt March.

“There’s all these things that give you the chills,” Gerwig says of writing and directing the film and trying to do Alcott’s classic work justice. “I felt like as soon as I give these lines to these actors, they brought them to life exactly how I was hearing them. It was almost like they were ringing out clear as the bells to me. It wasn’t like one thing that pushed it all into place. It was like I kept having revelations again and again that were not my revelations. They were Louisa May Alcott’s revelations that I could hear.”

In crafting the film’s first trailer, which debuted Aug. 13, Gerwig says that she “wanted it to feel like the movie feels, which is both classical and fresh. But it’s really about these characters and these relationships that have meant so much to me. And then it’s also infused with this mutual energy and this exuberance. And I think that that’s what the actors brought to the film and that’s what cinematography captured.”

Little Women opens in theaters Dec. 25.