In the film, Gosling and Williams spark a love connection.
Told in part with flashbacks, the movie documents their romance: the good, the bad and the ugly, as their relationship unravels in front of the viewer's eyes.
"I loved this script," Williams said. "I thought it was the best script I ever read and then I show up to work and they tell me, 'I want to throw the script away. I wrote that 12 years ago. It's dead.'"
So they tossed out the writer-director's script and did a lot of improvising instead.
It's was physically and sometimes emotionally draining.
"We just shot non-stop," Gosling said. "We would shoot like sundown, all night. The director would follow us around. Whatever happened, happened, and then sunup.
"I've wanted to make this movie for eight years and so whatever cost or toll it takes emotionally, there is a flip side to it to have waited so long for something and to finally see it coming together," Williams said.
When it all did come together, it initially got an NC-17 rating for its sexual content.
On appeal, the Motion Picture ratings board changed it to a more commercial R rating.
"Blue Valentine" hits theatres on New Year's Eve.