The cast of the new movie I Am Somebody is made up entirely of film extras, but you may never have guessed it when you watch one particularly heartbreaking performance in the movie.

China actor Shen Kai was so stunningly good in that scene that it would be easy to assume he was a veteran thespian: He is seen completely fearful and on the verge of a nervous breakdown because he repeatedly fumbles his lines in archaic language playing a minister in a period palace drama.

In fact, credit is due director Derek Yee for drawing that convincing performance out of the actor.

"Shen Kai had to memorise very difficult lines for his character. On the morning of the shoot, we swopped his script and gave him another set of complex lines to say.

"Of course he panicked. I just kept scolding him and had the cameras secretly rolling the whole time. His fear was absolutely real," Yee, 57, says in Mandarin with a chuckle.

What he did sounds cruel, but the Hong Kong film-maker is unapologetic about his ruse, which he says he resorted to with the best intentions.

"When you are working with a cast of completely inexperienced actors, you may have to employ such measures to get better performances out of them," he explains.

His new feature film is a labour of love, a tribute to the movie extras of China such as Shen who are so crucial to productions yet overlooked at the same time.

Centred on the lives of a group of colourful movie extras, the fictional work is set in the Chinese city of Hengdian, where thousands of people from all over the country move to every year in the hope of becoming a star.

Hengdian is home to Hengdian World Studios, said to be the largest movie lot in the world: Measuring 330ha, it is bigger than Hollywood's Universal Studios and Paramount Studios combined.

"I started talking to a bunch of these film extras in Hengdian a few years ago and I was just fascinated. They are like floaters, who drift from their hometowns to Hengdian with lots of hope, but no real direction. I knew then I wanted to make a film about them," he says.

Yee, who also penned the script, adds that since his work is all about film extras, it would only be right to cast them in all the main roles too. The central characters played by unknowns such as Wan Guopeng and Wang Ting were selected through auditions sifting thousands of hopefuls from across China.

While established stars such as actors Daniel Wu and Stephen Fung - not to mention directors Alan Mak and Felix Cheong - were given cameo roles, their casting was crucial to attract funding.

Yee, a veteran director behind award-winning films such as One Nite In Mongkok (2004), Protege (2007) and Shinjuku Incident (2009), says: "Without the big-star names fronting the project, it may be tough for the film to sell. In fact, if I had not been the director for this project, it may never even have gotten the funding to be made and made available for a wide release.

"Luckily, I had many good friends who were also willing to help publicise this movie."

He is referring to top stars such as Shu Qi, Brigitte Lin Ching-hsia and Tony Leung Chiu Wai, who took to social media to encourage their fans to watch the film after catching an early preview of the film. Leung's piece, in particular, was a lengthy 2,000-character essay about the film and its issues.

Yee says: "I asked them if they could share some thoughts with the public after they saw the movie and they were more than happy to help me. I am really very grateful."

As much as he empathises with movie extras, he stresses that it is a tough pathway to stardom.

"That's partly also why I made this movie. I hope when young people watch this, they will realise how difficult show business can be and not have any unrealistic expectations of becoming stars."

• Follow Yip Wai Yee on Twitter @STyipwaiyee

• I Am Somebody opens in cinemas tomorrow.