Patrick Schwarzenegger Was 'Almost Speechless' at Colin Firth's Transformation for 'The Staircase'

Reflecting on HBO Max's true crime shocker, Schwarzenegger tells PEOPLE his "career is just really starting" and that he'll "forever be grateful" for Firth's mentorship on and off screen

Photo: Courtesy of HBO Max

Patrick Schwarzenegger knew he had to be part of HBO Max's The Staircase as soon as he heard about it.

Speaking with PEOPLE, the 28-year-old actor opens up about his recent role as Todd Peterson on the true crime series and how working with the talented array of stars — including Colin Firth, Toni Collette, Dane DeHaan and Sophie Turner — was something he couldn't pass up.

"I was like, 'Holy s---, I've got to do everything I can to get this,'" he says. "To be surrounded by this kind of talent and these level of actors and actresses was something that was like a dream come true for me."

Based on the 2004 true crime docuseries of the same name, the limited series follows Firth as Michael Peterson, who was convicted of murdering his wife Kathleen Peterson (Collette) after she was found dead at the bottom of the staircase of their North Carolina home.

Having previously watched the docuseries with his girlfriend, Abby Champion, and mom, Maria Shriver, Schwarzenegger says he was intrigued by how the HBO Max show "went so much more in-depth with the [Peterson] family."

He adds that he was especially interested in the contrast between Michael's sons, Todd and Clayton (DeHaan). "Although they're brothers, they're very different in real life and in the show," he explains.

"I love Dane to death, but we're both very different people. That's what draws this dichotomy between the two characters and it makes it very evident on the screen. There's a very stark difference between [them]. I could never have played Clayton, and Dane was perfect for him."

Courtesy of HBO Max

While Schwarzenegger notes that some cast members reached out to the real-life people to inform their roles, he decided against it as he wanted to bring "[his] own interpretation" to the character.

That being said, he did plenty of research and preparation beforehand. "I had binders and folders online of videos and documents about Todd," he says. "I worked with movement coaches to embody his physicality and the way that he walked [and] how he talked."

He adds that the show's creator Antonio Campos actually had the cast do dreamscape work, which he says was like "going to therapy as your character."

"That was something that was new and enlightening for me and something that allowed me to grow a lot clearer perspective with other characters in the show," he adds.

While Schwarzenegger was in great company on the show, he says that working with Firth was the true highlight of filming.

Courtesy of HBO Max

"It was ridiculous," he says. "I would tell my mom and my dad [Arnold Schwarzenegger] when we were filming, 'Colin Firth is really insanely talented.' After getting to know Firth off-screen before filming, he admits he was shocked when Firth first jumped into character.

"I remember the first day we had scenes, he tapped into this character and I almost was taken aback of just like, 'Holy s---,'" he says. "I was almost speechless when we first worked together, I just couldn't believe how much of a transformation there was."

"I continue to speak highly of Colin and I will forever," he adds. "Because he's brilliant on the screen, but I've also talked about how great he was off the screen towards me and just welcoming me in. He was so helpful and willing to work with me so I'll forever be grateful."

Though Schwarzenegger already has a few big projects under his belt, including his upcoming role on The Boys spinoff and The Terminal List with brother-in-law Chris Pratt, he admits that it feels like "[his] career is just really starting."

"I'm still learning a lot and getting my feet wet," he adds, noting that his role on The Boys spinoff feels like the "perfect next step" in his career. "It's just a mix of action and comedy and some drama elements as well, so that's going to be a totally different genre."

In whatever he takes on next, Schwarzenegger says he always tries to follow his dad's advice to pursue what you're passionate about. "It's just being very determined and having a clear vision of where you want to go and not letting anybody else tell you otherwise. Continuing towards those goals is something that my dad always preaches to me."

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New episodes of The Staircase stream every Thursday on HBO Max.