Actor Alden Ehrenreich spent five years "auditioning" for Warren Beatty's new period movie Rules Don't Apply.

The Hail, Caesar! star reveals he had "hundreds" of meetings with actor-turned-director Warren over the process of developing the film, which the Hollywood veteran had started working on more than 20 years ago.

The lengthy "audition" turned into a mentorship of sorts for Alden, and he is grateful for the time he spent learning from Warren.

"I met with him for the first time when I was 19, in 2009, and we met for about five years in sort of his audition process, I guess," he explained to breakfast show Today. "But over that time I kind of had this apprenticeship with him, where I got to learn and ask him everything I ever wanted to know, so it was pretty great."

Rules Don't Apply stars Lily Collins as a young actress who embarks on a forbidden relationship with her driver, played by Alden.

Lily didn't have to wait quite so long to be cast in the film, but the Mirror, Mirror beauty admits it was an odd journey from being contacted by Warren to landing her role as Marla Mabrey.

"It was the most abnormal thing I've ever been a part of!" she laughed. "I basically just got a phone call from my agent saying that Warren Beatty wanted me to call him and gave me his number. I called him at home and it kinda just escalated from there. He came over the next day and we had a meeting, (at) which we didn't talk about the movie at all, and then it progressed to four or five lunches and dinners with Annette (Bening, his actress wife) at the house and family - didn't talk about the script for the fourth or fifth meeting, and then finally read the script!"

"I got off easy, it was a couple months," she added.

Lily reveals working with Warren was "intimidating", but in the best sense of the word: "He's a meticulous guy, he knows what he wants, but he's a genius and you kinda have to... allow him to do his thing," she explained.

And the best piece of advice she received from Warren was to "let it go".

"He said, 'Be in the moment, surprise yourself, allow yourself to surprise others and if you can go home at the end of the night and just feel proud that you almost don't remember what it is that you did but it felt right, that's the best thing,'" she recalled.