Together again: Why Emma Stone and Ryan Gosling are so good together

They became one of our favourite onscreen couples in the 2011 comedy Crazy, Stupid, Love, before teaming up again in the film noir homage Gangster Squad.

Now, the world is set to fall in love with Ryan Gosling and Emma Stone again, as they channel another Hollywood power couple, Fred Astaire and Ginger Rogers.

Like Fred and Ginger, the pair have never been romantically linked in real life. But their chemistry lights up the screen, with many already picking La La Land as the frontrunner for best picture at next year's Oscars.

The actors had to dust off their dancing shoes for the musical.

"It was wonderful to get to work with someone you know and trust and respect so much as an actor and as a human being," Stone says in her deep husky tones.

"I knew Ryan could sing and dance but we had to learn to ballroom dance together. Once you've learned to ballroom dance with somebody, you've learned everything you need to know!"

After opening the Venice Film Festival in September, Stone picked up the prize for best actress, and both she and Gosling earned Golden Globe nominations last week for their roles.

But it's not just the leads who make it so special. Behind the camera, the film's success stems from the genius of its 31-year-old writer-director, Damien Chazelle, and his former Harvard roommate, composer Justin Hurwitz, who previously collaborated on 2014's Whiplash.

Ryan Gosling and Emma Stone are both up for Golden Globes for La La Land.

Chazelle had written his LA-set romance about a struggling jazz pianist and aspiring actress four years before he made smash hit Whiplash, which drew on his tortured experience as a jazz drummer.

"For many reasons I just wasn't good enough," the 31 year old admits. "I remember every rejection letter I've ever gotten and that works its way into any screenplay I write. I try to make all my movies as personal as possible."

Of course La La Land had been one of those rejections. Musicals had been deemed dead in the water by the major studios and too expensive for everyone else, so he went ahead with the US$3.3 million Whiplash instead.

Whiplash opened at Sundance to a euphoric crowd, injecting new life into the careers of Miles Teller and J.K. Simmons, who won the supporting actor Oscar, and claiming a total of five Oscar nods.

Emma Stone and Ryan Gosling star in La La Land.

Following that success, La La Land was quick to get the greenlight. But with a budget 10 times the size, Chazelle admits "it was still a big gamble".

Stone and Gosling have also experienced the trials and tribulations of trying to make it in Hollywood.

"I have a few humiliating audition stories," she admits. "There are so many different types of people who can crush your dreams or help them come true. It never quite got to the point for me where I might pack it in, yet I don't feel I was ever as brave as Mia is in putting herself out there in that way.

"Mia feels there's something special inside her, but she doesn't quite know what it is. It was exciting to take her into this musical world where you can suddenly spin down the street or burst into song."

Ryan Gosling and Emma Stone have played on screen lovers in three separate films.

Having just starred as Sally Bowles in Cabaret on Broadway, Stone came to the production ready to warble. Gosling needed a little more time to prep, undertaking an intense three-month training regime to play the role of a professional pianist.

"I always like the opportunity to learn a new skill, or refine one," he says. "We had wonderful coaches and really beautiful music to do it to. I've now played the theme more times than I can say but I never get sick of it."

Chazelle had planned on hiring a pianist double but realised there was no need. Gosling's proficiency even stunned 10-time Grammy Award winner John Legend, who has his first speaking part in the film.

"I hope I didn't embarrass myself," Legend says. "I was kind of jealous about how fast Ryan learned to be awesome at playing piano. I've been playing since I was 4."

Legend, who proves he is no slouch in the acting department, plays a jazz performer who recruits Gosling's character Sebastian to play in his band.

"I felt like I understood Sebastian a lot because I've been around musicians my whole life and I understand those who are more pure and bound to tradition and don't want to sell out," Legend says. "I'm kind of like a sell-out in some ways," he laughs, "because you know, I've done okay."

Gosling insists Legend is being modest about his acting ability. "[There's a] wonderful speech he has in the middle of the film about pushing the art form forward versus selling out. John improvised that whole speech and it's beautiful."

LOWDOWN:
Who: Emma Stone and Ryan Gosling
What: La La Land
When: In cinemas Boxing Day