United Kingdom, March 24 -- Reese Witherspoon and Harlan Coben are "obsessed" with their novel.

The 'Big Little Lies' star announced in October that she was teaming up with the 'Fool Me Once' author for her first foray into fiction, but the 63-year-old writer admitted he initially rejected the idea of a collaboration, only to embrace the project wholeheartedly when the 49-year-old actress shared her ideas with him.

Harlan told You magazine: "People often suggest collaborations, but I say, 'Respectfully, that's not for me.'

"I thought, 'How do I nicely tell Reese Witherspoon, who I love, this is a no?' But when she told me her idea, I was quickly thinking, 'OK, maybe if we move this bit here and that bit over there.'

"Now we are obsess...