It. Will. Rock. Your. World.

"The Notebook" is one of those movies where the relationship seems so real, it even makes MEN cry.  And believe it or not, last month was the ten-year anniversary of it.

But we're just now hearing that RYAN GOSLING and RACHEL MCADAMS' chemistry on-set WASN'T all that great.  In a new interview with VH1, director NICK CASSAVETES reveals that, in fact . . . they kind of HATED each other.

Cassavetes says the worst day of filming was when Ryan asked him to kick Rachel OFF THE SET, and bring in another actress to practice his lines with . . . because, quote, "I can't do it with her.  I'm just not getting anything from [her performance]."

Which Rachel apparently did NOT appreciate.  Because they went into a room and SCREAMED at each other for a while, until they eventually hashed things out.

Apparently it got better after that, but they STILL didn't always get along.  According to Cassavetes, quote, "The rest of the film WASN'T smooth sailing, but it was SMOOTHER sailing."

Which is interesting, because Ryan and Rachel eventually went on to date in REAL LIFE for two years . . . from 2005 to 2007, and then again in 2008.

It's not clear how they were able to turn that lack of passion into a RELATIONSHIP.  But in 2007, Ryan talked about people comparing them to the characters in the movie.

He told "GQ", quote, "People do [us] a disservice by assuming we were anything like the people in that movie.  [Our] love story is a hell of a lot more romantic."