The 8 biggest differences between the 'It Ends With Us' book and movie

In Hoover's book, Lily is 23 years old and a recent college graduate when she first meets 30-year-old Ryle.

Her other love interest, Atlas, is described as two-and-a-half years older than her.

But in the film, the characters are played by actors significantly older than them — Lively is 36, Baldoni is 40, and Sklenar is 34.

Hoover has previously said that she "messed up" by making the characters in the book too young.

She has said that her involvement in the movie allowed her to correct her mistake.

"Back when I wrote 'It Ends With Us,' the new adult (genre) was very popular," she said, per Today. "You were writing college-age characters. That's what I was contracted to do. I made Lily very young. I didn't know that neurosurgeons went to school for 50 years. There's not a 20-something neurosurgeon."

In the book, readers see Lily and Ryle's will-they-won't-they romance play out over several months after they reconnect when Ryle's sister, Alyssa (Jenny Slate), gets a job in Lily's flower shop. Lily sprains her ankle one day, leading Alyssa to call on her brother for help.

Ryle repeatedly pleads with Lily to agree to a one-night stand. While Lily initially is steadfast in her refusal to be another one of Ryle's girls, she eventually decides to go for it. However, an exhausted Ryle then falls asleep in Lily's bed.

This leads to frustration on both sides, which comes to a head on the night of Alyssa's birthday. The two agree to compromise and have a trial run of a relationship. They go to bed together but don't sleep together.

Things play out a little differently in the movie. After Ryle and Lily are reintroduced at the flower shop, the pair join Alyssa and her husband Marshall (Hasan Minhaj) on a night out in Boston involving watching a Bruins game, karaoke, and bowling.

Some flirting ensues and at the end of the night, Lily agrees to kiss Ryle after he says that it's all he needs from her to stop him pursuing her further. After this, the pair continue to flirt and end Alyssa's birthday as in the book.

It would be an understatement to say that former TV staple Ellen DeGeneres has a significant role in the book, with Lily harboring an obsession with the TV host.

Once Atlas begins hanging out at her house after school, the two teens bond over watching the show together, and a quote from DeGeneres's "Finding Nemo" character — "Just keep swimming" — becomes their way to comfort each other during difficult times.

All of this is downplayed in the movie, however, with almost all references to DeGeneres removed.

In both the book and the movie, Atlas moves into the abandoned house backing onto the Bloom property after being kicked out of his mom's house.

In the book, Atlas tells Lily that he's also a victim of abuse; his stepfather burned him with cigarettes, and his mother didn't stand up for him. When he tried to return home after running away, his mom told him to stay away as it would upset his stepfather too much.

However, the book doesn't mention the abuse Atlas has suffered at the hands of his stepfather. Instead, when Atlas confronts Lily in the restroom of his restaurant, it's implied that his mom, like Lily's, was abused by her husband.

In the film, Ryle's proposal comes on the same day Alyssa gives birth. Ryle realizes that he wants to spend his future with Lily and gets down on one knee to propose to Lily in the hospital room.

Book readers will know that the night that Alyssa's baby is born ends differently for the couple.

Already married at this point in the book — they elope to Vegas — Lily returns home to a seething Ryle who has read through all her teenage journal entries. His verbal and physical attack on her at this point becomes her breaking point, and she takes refuge at Atlas's house.

Atlas's restaurant in the book is named Bib's, which stands for Better in Boston. This phrase is written on the souvenir magnet Atlas gave Lily when they were teenagers.

The restaurant has been renamed Root in the movie. However, it's still an homage to Lily, as audiences see a flashback in which a teenage Lily teaches Atlas about the importance of plant roots.

The revelation that Ryle accidentally shot and killed his brother when they were young children is one of the book's most shocking twists.

Ryle opens up about the traumatic incident, which he says causes his rage-filled episodes after he physically abuses Lily.

Feeling sympathy for him, Lily decides to forgive him, and the two discuss how Ryle can work through his anger.

In the film, Alyssa fills Lily in on the Kincaid family's dark secret only after Ryle's third and final attack on Lily. By this point, the pair have separated.

In the book, Lily doesn't know the gender of her baby until she's holding her newborn in her arms, which makes her decision to ask Ryle for a divorce moments later even more poignant.

It's not until it dawns on her that she has a daughter who could grow up watching her mother be abused by her father that she decides to end the generational cycle of abuse.

This realization leads to the film's emotional climax, in which Lily breaks Ryle's and her own hearts to protect her daughter.

In the film, both of them know that they are going to have a daughter beforehand, which plays down the emotional gravitas of the scene.