Los Angeles (CNN) Damien Chazelle spent six long years trying to make his musical love story, "La La Land." But these days, he's thankful for the bumps along the way because they all led him to the two stars who would make his movie a reality -- Ryan Gosling and Emma Stone.

"Ryan and Emma coming on board felt like one of those fortuitous, chance things," director Chazelle told a crowd at Entertainment Weekly's PopFest Saturday afternoon. "At that moment, the project suddenly became real in a sense for the first time in six years [and] the wheels started turning."

In the film , opening December 9, Stone and Gosling play two Angelinos whose dreams and mutual romantic feeling collide in retro Hollywood musical form. Early buzz out of the major film festivals puts the movie as a frontrunner at the Oscars.

Along Chazelle's path to "La La Land" were money issues and casting struggles, but with Stone and Gosling in place, he said, "we hit the ground running."

The film's stars were involved with prep from the start because their roles required training and rehearsals.

"It was this really hands-on three or four month period where we were more or less in each other's faces every day," Chazelle joked.

Chazelle took part in a panel that included filmmakers of other possible Oscar contenders, including Barry Jenkins ("Moonlight"), Pablo Larraín ("Jackie"), David Mackenzie ("Hell or High Water"), and Mike Mills ("20th Century Women").

Mills said casting was also key in his film's result, but it came with added pressure. The main character in "20th Century Women," eventually played by Annette Bening, is based on his mother.

"I knew Annette could hold all the complexity of and the humor and the heaviness," Mill said. "But the funnest part for me was giving the keys to my mom's car to Annette."

As a man, Mills said, "I don't know how to be a middle age woman," so he told Bening early on that it would be her "heart and psyche and brains that are going to pull this off."

"20th Century Women" opens December 25.

Larraín had no shortage of contenders when it came time to cast Jacqueline Kennedy for "Jackie," a movie that shows the former First Lady in the days after her husband's death. But it was star Natalie Portman who he said was the only one who could "combine beauty and sadness in a really powerful way."

"I'm sure many actresses could play a great Jackie, but to me, it was always Natalie," he said. "Not just because she has the elegance and sophistication but also I think Jackie was someone who had a lot of mystery."

The main role in coming-of-age drama "Moonlight, " too, called for an actor who could portray nuanced duality. Jenkins luckily found just what he needed in Trevante Rhodes, a man who has "muscles on top of muscles" but a sensitive center, the director said.

"It sort of changed the idea of what we were talking about ... this idea of masculinity run amok," Jenkins said.

"Moonlight" is in theaters now.

EW's PopFest continues through Sunday in Los Angeles.