Inside Blake Lively and Jenny Slate's “It Ends With Us” Friendship 15 Years After Meeting on “SNL” (Exclusive)

Jenny Slate says she "lucked out" getting to work with costar Blake Lively

Blake Lively and Jenny Slate formed a bond playing best friends in It Ends With Us, but it wasn't the first time their paths had crossed.

"We met when you were hosting SNL," Slate recalls to Lively during an interview with PEOPLE in New York City on Aug. 4. Slate was a Saturday Night Live cast member in 2009 when Lively hosted the show.

Continues Slate, "I very much remember you were very, very nice. I think we were doing the Muppets or something for your opening number?" Lively says, "Yes. That was so fun. You were the one with the long hair."

"I was Janice, yes!" says Slate. "And you had a strapless dress. This might be wrong, but I remember being like, 'Oh, you’ve got to get your dress up,' and I remember you being like, 'Thanks!' "

Lively remembers, "It was so sweet of you to take care of me, though, because I didn’t have a stylist and I bought a dress at Topshop. I was like, 'I’m hosting SNL, I guess I need a dress!' But it probably should have had a whole thing, but it wasn’t tailored properly."

Related: Blake Lively Reveals the Romantic Gesture Ryan Reynolds Did Every Week When They Began Dating (Exclusive)

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In It Ends With Us, the new romantic drama that brings Colleen Hoover's bestselling novel to the big screen, Lively plays Lily Bloom, while Slate plays Allysa, who becomes her BFF and employee at her newly opened flower shop in Boston.

Lively says her comedian costar is "just so interesting." She says of Slate, "You can talk to her for hours and hours and be infinitely interested. Yet she's also someone who what you see is what you get, in the most complimentary way. You're not like, oh, she presents as this, and then you get to know her and there's a totally different [side]."

"She's very authentic. There's no other side of Jenny. There's just a deeper, deeper well of this one," adds Lively. "... She's so sweet but she's got edge. There's not a mean bone in your body."

Slate explains that there were "so many different things for us to connect on, especially being parents." Lively has four kids with husband Ryan Reynolds, while Slate has one child with husband Ben Shattuck. Says Slate, "Going to work and still having our heart in our work but also with our children — that is a very big challenge."

Plus, she adds, "My friends have consistently been my actual heroes because they’ve offered stability and light in times of darkness or complication. Getting to work with Blake and build that bond on this film is something I will always be grateful for."

"Allysa and Lily have a true bond, human to human, and it’s been such a gift that Blake and I developed that as well," says Slate. "I lucked out."

It Ends With Us is in theaters now.

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