The actress' decision to take on the critically-acclaimed drama "An Education" might seem like simple arithmetic now. But there was a long period of time when it took a leap of faith to see if the project was even going to get off the ground.
"We ran around the houses for a bit," Hornby told the "Creative Screenwriting" screening at the L.A. Film school on Sunday.
Even landing the backing of a big fish such as the BBC was only part of the battle. "They can't pay for a whole movie. They can pay for a quarter of it. They need co-producing partners."
In the meantime, Hornby had to keep his cast together. "Peter (Sarsgaard) was on board at quite an early stage and stuck around for a while. He was quite tenacious about it," says Hornby. "Emma (Thompson) had kindly agreed to do that turn to help us out."
But the key to the puzzle was the central role of the sheltered teenager who has the affair with the much older man.
"We'd seen Carey and a couple of other people. But we knew that she was the one," says Hornby.
"I was never worried about Carey. I said to her, you'll get much better parts than this, but you'll never get a bigger one. She has a speech on every page of a 120-page screenplay. Actresses like that."
Muligan stuck around, and it paid off. She received rave reviews and is now up for a Best Actress Oscar.