Hoa Xuande was unsure about his ability

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 Prizewinning 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 VietnameseAustralian 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 watch "I made him 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.