The It Ends With Us movie excitement has only just begun.

And author Colleen Hoover knows fans (and critics) have strong opinions about her 2016 best-seller, and what parts of the intense story made it—or didn't make it—into the film starring Blake Lively and Justin Baldoni, who also directed.

"With every adaptation, things have to change," Hoover told E! News in an exclusive interview. "You're trying to condense this whole storyline into a film, so there were some changes to make the flow better."

We know that the fractured relationship at the heart of the book is intact: Lively plays Lily Bloom, who finds herself rationalizing her volatile lover's abuse—much like her own mom, who suffered in silence at the hands of her father.

But as Hoover said, not everything on the page made it onscreen.

For starters, the restaurant owned by Atlas—the man who deserves Lily's heart, played by Brandon Sklenar—is no longer called Bib's.

"It stood for something in the book that became a big thing for Lily to show how much she meant to him," Hoover explained. "So that changed to Root in the movie because we didn't have as much time to put in all of the things that happened in the book to make that significant moment happen in the movie."

However, she added, "changing it to Root went back to a conversation they had as kids in the film [from which] you get the same feels that you got in the book. It's just completely different. And then, obviously, the ages."

The book begins with Lily freshly out of college and headed to Boston to realize her dream of opening a flower shop. And while we wouldn't have fought anyone had the movie tried to pass 36-year-old Lively off as a recent grad, the filmmakers purposely wanted a more seasoned star.

"We aged up the characters," Hoover said. "Eight years ago, when I wrote the book, new adult was huge. And everyone was wanting to read characters in their very early 20s. And this is such a tough subject matter that putting young, young characters on screen just didn't feel right to us. So I think aging them up was such a great move, and it worked so well on film."

Not least because of their leading lady.

When Hoover was informed that Lively was onboard—"I wish I could say that I had anything to do with her casting"—the writer was cautiously thrilled.

"I didn't want to get my hopes up," Hoover said, "because I was so excited about the possibility of her playing this role. When it actually happened, and she showed up to set and started filming, it was so surreal. But she is phenomenal."

And that trolling-meets-true-love dynamic Lively brings to social media with her real-life husband, Ryan Reynolds?

"The humor that you see between her and Ryan online, she brings so much of that to the character," Hoover said appreciatively. "With the subject matter and the emotions of this book, it needs that comedic levity, and she does it so well."

An important detail that Hoover really wanted to see in the movie—though, she added, "I tried to let them make the movie that they wanted to make"—was Lily's heart tattoo.

In addition to countless readers who've had the symbol of the open-ended love in Lily's life inked into their own flesh, "My mom and my sisters and I have the tattoos," Hoover shared. "So that's one of the things that I was super-excited that made it in there."

But more meaningful to the Texas native than anything else was that the movie kept her ending "almost word for word."

It's "a very faithful adaptation," Hoover said. And "there was one scene that I really fought for from the book," and one line in particular she hoped would make it. 

P.S. It made the cut. 

Of course, none of it would work without the right actors opposite Lively. Hoover thought Sklenar was the perfect choice for Atlas ("He brings that kindness and protectiveness"), and she sang the praises of Baldoni, who bought the rights to the book years ago but didn't automatically see himself playing Ryle Kincaid.

He was "playing around with the idea" of directing and starring, Hoover said, but it wasn't until they began the casting process that it became apparent the 40-year-old should pull double duty.

"It it all just worked out to where he was wanting that challenge and wanting to take on that role," Hoover said. "And he did such a phenomenal job—both of them together, their chemistry on screen. It's great."

Overall, the story has been brought to uncanny life for its author, whose 2018 book Verity is next in line to become a film.

"After watching the movie and writing these roles," Hoover said, "the casting is so spot-on and everything I was hoping this movie would turn out to be."

It Ends With Us opens in theaters Aug. 9. And here's everything else we know so far about the film:

Gird your loins as pop culture's next great love triangle is about to begin. Based on Colleen Hoover's bestselling 2016 novel of the same name, which dominated TikTok in 2022, It Ends With Us is bringing to life the romance drama centering on a beautiful florist. 

"Though coming from a complicated past, Lily Bloom has always known the life she wants," the film's synopsis read. "While living in Boston, she meets neurosurgeon Ryle Kincaid and believes she may very well have found her soul mate. Soon, however, questions arise about their relationship, and to complicate matters her high school love interest, Atlas Corrigan, comes back into the picture, putting her relationship with Ryle in jeopardy."

So, which actors are portraying Lily, Ryle and Atlas? 

"A flower that's still blooming."

That is how the film's official Instagram account described the Gossip Girl alum's character, who is the florist at the center of a love triangle. To portray the beloved character, Blake dyed her signature blonde tresses red. Fans got their first look at Blake's fiery new hue when she was spotted on the New York-area set.

Not only will Justin Baldoni portray Lily's complicated doctor love interest, but the Jane the Virgin star is also directing the movie. 

As for the rest of the behind-the-scenes team, both Justin and Blake as well as Colleen will have creative say as executive producers. Other producers include Alex Saks (The Florida Project) and Christy Hall (I Am Not Okay With This), who is writing the script, per a press release from Wayfarer Studios and Sony Pictures.

Blake isn't the only cast member changing up her hairstyle for her character as Justin chopped off his long hair prior to filming to achieve Ryle's clean-cut look. Justin documented his haircut in an April 15 Instagram video, revealing he was donating his strands to Locks of Love.

In addition to cutting his hair, Justin also revealed he was hitting the gym. "A lot of change in one week," Justin captioned an April 17 gym selfie. "Cut my hair. Started eating carbs. And now I'm seeking validation from strangers on the internet by posting thirst traps. Wait …is this a thirst trap? Just Googled it. Don't google thirst trap."

Brandon Sklenar—star of the Yellowstone spinoff 1923—is set to play Atlas, Lily's childhood friend and former flame who becomes a romantic rival for Ryle. 

Justin confirmed Brandon's casting on Twitter April 20, writing, "Ladies and gentleman… meet your #Atlas."

After the news was announced, Brandon took to Instagram to share a heartfelt message about joining the movie.

"When this project came along I wasn't aware of this book or the tremendous impact it has had on women all over the world in helping them to have the strength to change their lives," he wrote on April 21. "This is something very personal to me. The amount of women I've known who have been victims of abuse is staggering. It is our intention that this film continues to spread the vital message of this book; for women to feel empowered and men to be better. I'm honored to be a part of it."

It was announced May 1 that Jenny Slate had joined the film's roster as Allysa, Ryle's sister and Lily's best friend. "Pinch me," Justin wrote on Instagram of the Saturday Night Live alum's casting. "Hey sis!"

Describing Jenny as "one of the most talented humans," Colleen admitted on social media that she "can't even believe she'll be in this movie."

The actress picked to portray a younger version of Blake is Isabela Ferrer, according to IMDb.

Justin confirmed in April 2023 that they had found their star following an extensive search that included an open casting call on social media."Thank you for being a part of the process, it was inspiring and all of the messages we received was just validation for why we're making this movie and why this movie is so important," he shared on Instagram. "We found our young Lily, we'll be announcing her soon."