LOS ANGELES – Some three months into shooting The Sympathizer, American actor Robert Downey Jr sat Hoa Xuande down. He had something to show him.

“I remember Rob walking in – he had this cheeky grin,” Xuande recalled on a recent afternoon in Los Angeles.

A teaser trailer for the HBO black comedy series, an adaptation of Viet Thanh Nguyen’s 2015 Pulitzer Prize-winning novel, had just been cut. Downey, who is the show’s executive producer and plays multiple roles in it, saw definitive proof of a star-making turn in The Sympathizer, now available on HBO Go.

He wanted Xuande, the star in question, to see it too.

“There’s only one time that I’ve had this experience before, and it’s when I saw the teaser that we brought to Comic-Con for Iron Man,” Downey, 59, said. Seeing himself on-screen in the Iron Man suit was what finally convinced Downey that he had done justice to a daunting role.

“And because I’d had that experience,” he said, “I knew that he needed it.”

In many ways, Xuande (pronounced Shawn-day) did.

A 36-year-old Vietnamese-Australian actor who had one Hollywood credit to his name, he still was not sure he was the right choice to lead a series with such an impressive pedigree: an HBO adaptation of an acclaimed novel, produced by the Oscar-winning art-house studio A24, directed by the revered South Korean auteur Park Chan-wook and co-starring a screen legend in Downey.

He needed all the encouragement he could get.

“I made him watch it six times,” Downey said.

Seeing himself in the trailer had finally quieted his doubts, Xuande said, over lunch at a Venice restaurant. He had flown in from his home base in Sydney hours earlier and had barely settled into Downey’s spare live-work space, where Xuande sometimes stays.

He first met his co-star on the fourth day of shooting – Downey was wearing a bulging prosthetic nose and had a skittering Yorkshire terrier in tow.

Is that really him? Xuande remembered wondering from afar.

Downey had arrived on set in costume for one of the five characters he plays in The Sympathizer, including a cartoonish professor, an empty-suited congressman and a gonzo film-maker – different people who together embody the face of the American establishment.

On this day, Downey was Claude, the CIA handler for Xuande’s character, who is known as the Captain, a nameless communist spy whose undercover assignment leads him to Los Angeles after the fall of Saigon in 1975.

His role as a double agent in the United States becomes increasingly dicey, bloodying his hands and landing him, for reasons the viewer does not quite know, in the Vietnamese re-education camp where the show opens.

“He made me feel really at home, as much as I could feel,” Xuande said of that first encounter. “And then he just put his hand on my shoulder. He was, like, ‘Buddy, you and me, we’re going to sc*** this up together.’”

It put Xuande at ease – slightly. Before he got the role that could change his life, he had already let it go many times over.

He was chosen through a worldwide casting call and a months-long audition process that included long stretches of radio silence. During those times, he would take night-time strolls from his home to Sydney Harbour, where he would sit for hours, making peace with the fact that he probably had not got the part.

Finally landing it was a profound professional leap: Beyond a side role on Netflix’s live-action anime adaptation Cowboy Bebop (2021), he had spent a decade taking minor parts in Australia. Hollywood had been a faraway notion.

Xuande grew up in Melbourne, the son of bakers who had fled post-war Vietnam.

Naturally, something as fanciful as acting was not on Xuande’s radar and, for years, he dreamt instead of playing Australian rules football professionally.

After moving to Sydney for college and juggling odd jobs in his early 20s, he fell in with a group of creative friends and began doing amateur theatre. A director saw his potential and pushed him to get training, leading Xuande to the Western Australia Academy of Performing Arts in Perth.

After several years of hustling in mostly bit parts, he had a breakthrough with Bebop, his introduction to the machine of a major Hollywood production.

The Sympathizer, though, was a trial by fire.

Because of visa issues, he was unable to join the production until just a week before the intense six-month shoot began. It took place mostly in Los Angeles and Thailand.

After a crash course to upgrade his Vietnamese, he found himself on set acting opposite Korean-American actress Sandra Oh, another award-winning co-star, and the many versions of Downey, under the direction of Park, who communicated via a translator.

It was a great deal of pressure for an actor new to Hollywood. But Oh, 52, who plays Ms Mori, the Captain’s love interest, said “if he was terrified, I never felt it”.

On set, she often stepped up for Xuande, demanding time or boundaries for a newcomer who was suddenly at the top of the call sheet. “Sandra mothered me – she would speak up on my behalf when I didn’t feel comfortable to,” Xuande said. “And Robert would father me. He would just look after me on the other side of things.”

Xuande is trying to stay levelheaded about his own exposure to the spotlight and what it means for his career.

He hopes The Sympathizer gets another season – Nguyen wrote a sequel, The Committed, published in 2021 – but Xuande knows the industry can be fickle.

“I hope that it leads to other opportunities,” he said. “But I’ve been saying to my friends that this could be it.”

He is joking – sort of. Even with such a big-ticket series under his belt, he still has the mentality of a gigging actor, climbing his way up.

“You never know: It could be months, years, in between work,” he said with a hint of the Captain’s smirk. “But I’m happy I got to do this.” NYTIMES

• The Sympathizer is available on HBO Go.