'Dash & Lily's Book Of Dares' Authors Wonder How They Can Cast Justin Bieber

It was probably no coincidence that on Tuesday, the day Rachel Cohn and David Levithan's new novel, "Dash & Lily's Book of Dares," was released, it was announced that megaproducer Scott Rudin had optioned it for a movie. But the authors were super-excited about the news anyway, especially after having so much fun watching their first collaboration, "Nick & Norah's Infinite Playlist," turned into a movie starring Michael Cera and Kat Dennings.

"The first time we were on set, it was on Ludlow Street, on the Lower East Side," Rachel recalled. "And we turned a corner and there were these huge trucks everywhere and lighting being brought down from a crane, and it's just surreal to think of this little thing that we kind of started as a joke; for this one moment, there's 50 people running around doing all kinds of things. I was like, 'We are gods! We made this happen!'

"Dash & Lily," their third book together (after "Naomi and Ely's No Kiss List"), is a kind of treasure hunt through New York that begins when normally cautious, sweet Lily leaves a puzzle in a notebook on a shelf of the Strand bookstore in a ploy her brother devised to help her meet boys. Dash, a much more cynical, "snarly" type, finds the book and sends Lily his own puzzle. The two traipse around the city during Christmas break, sharing secrets and hopes without ever meeting each other.

Rachel and David wrote the book by trading off chapters, with no real plan in mind, the way they've always collaborated. Each writer ends a chapter with a dare for the other to take on in the next, much the way Lily and Dash trade dares. And after having one book adapted into a movie, David said he had another dare of his own to take on.

"I think one of the things the ['Nick & Norah'] movie does that the book doesn't have is a lot of humor and slapstick comedy," David said. "I think in writing this book, because it came out after that movie, I was like, 'OK, can a book have slapstick comedy?' So I think there are some very slapstick moments in the book. It wasn't because I was casting the movie in my head, going, 'Oh, this would be a great scene in a movie.' It was more a dare to see 'Can a book do something that a movie does?'"

Of course, we had to ask who they'd like to star in this new movie. They both would love Elaine Stritch, the Broadway star who plays Alec Baldwin's mother on "30 Rock," to play Lily's eccentric great aunt.

"But Dash and Lily, it's interesting, because I think in our head, we never cast it," David said.

"I literally have no idea," Rachel added. "We've been instructed by a young pre-teen we love that Justin Bieber may not be cast, but we can't do anything else about it."

"I think he would make a great Lily," David deadpanned.

As for the one scene they'd demand writer/director Lena Dunham keep in the movie, "Without giving anything away, I would say the flying baby scene," David said immediately.

"That would be mine too. The Crimson Alert, where David Levithan meets slapstick," Rachel agreed, before David had another inspiration: "Justin Bieber could play the baby!"

When you've read the book and figured out what the heck Rachel Cohn and David Levithan are joking about, share your own casting ideas in the comments!