The hugely popular film is celebrating its 10th anniversary, and Cassavetes told VH1 that behind-the-scenes drama helped make the movie a must-see classic.
He explained: "Maybe I'm not supposed to tell this story, but they were really not getting along one day on set. Really not. And Ryan came to me, and there's 150 people standing in this big scene, and he says, 'Nick, come here.'
"He's doing a scene with Rachel and he says, 'Would you take her out of here and bring in another actress to read off camera with me?' I said, 'What?' He says, 'I can't do it with her. I'm just not getting anything from this'."
The director insisted the two take a break and work out their problems in private. "We went into a room with a producer; they started screaming and yelling at each other. I walked out. At that point I was smoking cigarettes. I smoked a cigarette and everybody came out like, 'All right, let's do this'," Cassavetes added.
"It got better after that. They had it out... I think Ryan respected her for standing up for her character and Rachel was happy to get that out in the open. The rest of the film wasn't smooth sailing, but it was smoother sailing."
The on-screen tension resulted in an off-screen relationship. The pair dated from 2005-07 after filming wrapped and briefly reunited in 2008.
Gosling told GQ magazine before they split, "(The Notebook) introduced me to one of the great loves of my life. But people do Rachel and me a disservice by assuming we were anything like the people in that movie. Rachel and my love story is a hell of a lot more romantic than that."
The actor is currently involved with actress Eva Mendes, while McAdams is dating Canadian music manager Patrick Sambrook.