Sandra Oh 'Spoke Up' on Behalf of Hoa Xuande on Set of “The Sympathizer”: 'A Dream' (Exclusive)

Xuande tells PEOPLE that both of his costars, Oh and Robert Downey Jr., were major sources of support on set

Walking on set of The Sympathizer alongside Sandra Oh and Robert Downey Jr., Hoa Xuande wasn't sure what to expect.

The Australian actor, 36, who was nominated for a Gotham TV Award for his leading role on the HBO series, tells PEOPLE that he was happily surprised by the support he received from his costars — particularly Oh, 52.

"Sandra really stood by me throughout all of the shooting period because she knew what it was like for a young actor to start out," he says of Oh, who played Sofia Mori, a love interest for his character. "She really spoke up on my behalf when I felt like I couldn't. I really appreciated that."

Downey Jr., 59, was similarly easy to get along with — and offered some tips of his own for Xuande. "Robert is just the most funny, down to earth, self-deprecating, silly kind of guy to work with. He really made me feel like I was working with just another actor and he really was supportive and generous in helping me just be confident in my choices," he shares.

Related: Robert Downey Jr. Isn't Afraid to 'Get Funky' or Go Shirtless as He Shapeshifts Into 4 Villains in Sympathizer Trailer

"Honestly, to walk on set in a leading role and have two people who are professionals in their field to really back you, it was honestly amazing," Xuande adds of his costars.

"It's a dream. I've peaked. That's it. I can't do anything else after this. This is it."

Xuande played a spy solely referred to as "The Captain" in the limited series, which saw Downey Jr. take on multiple roles, including as an underground CIA agent who became Captain's boss when he fled Vietnam during the Vietnam War. Captain embarks on what Xuande calls a "deep and dark and hilarious journey" as he remains a spy for the Viet Cong, despite having fled the South Vietnam army to come to the U.S.

Aside from his costars being a major source of support — and fun — on set, there were challenges to taking on the complex story based on Viet Thanh Nguyen's novel of the same name. Xuande says the "biggest" challenge of playing Captain was the story's "nonlinear timeline."

"The show jumps around all over the place a lot of the time, but I think that was the rewarding thing about this story, is that it really allows people to watch and watch and re-watch it all over again because there's many little clues that are littered all over the place."

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The Sympathizer is now streaming on HBO Max.

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