The British movie, described as an anti-romantic comedy, is the story of a first year of marriage when the couple are wildly unsuited.
Writer and director Dan Mazer was worried that the pair's friendship resulted in too much "sparkle" on screen.
"I think Dan was nervous because we became friends pretty quickly," Rose, who plays an uptight and humourless high-flyer, said.
"As a director I can understand because that's not the dynamic he wanted. He'd say, 'You've got to be mean to him'!"
Rafe, who plays semi-successful novelist Josh, said he had to prove he was "handsome, funny and charismatic enough," for the role.
"The casting of a lead part is a huge responsibility, so I understand people being precious about it," he said.
:: I Give It A Year is released on Friday, February 8.