Colin Firth Says Toni Collette’s Performance in ‘The Staircase’ Shook Him (VIDEO)

We love a juicy true-crime case — especially one that involves a complicated prominent family with secrets! HBO Max’s eight-episode limited series The Staircase explores the real-life murder (or not?) of Kathleen Peterson (Toni Collette) and jumps through time to investigate just how involved her husband Michael Peterson (Colin Firth) was (or wasn’t) in her death.

As Collette and Firth tell TV Insider, the series may be placed in the true-crime genre but it’s the family drama that was a selling point for both actors. “[The family element] just gives it so much more grounding and heart. These bonds are real,” says Collette. Firth agrees that, while the potential crime is a big part of the story, “the mystery for me — and I think it’s full of mystery and I think it’s full of twists and turns and unpredictability — wasn’t dependent on the whodunit.”

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Instead, Firth adds, a lot of the series, which also stars Michael Stuhlbarg, Rosemarie DeWitt, Odessa Young, Parker Posey, Patrick Schwartzenegger, Dane DeHaan, and Sophie Turner, is about what we really know about each other in the family setting. “As soon as you feel certain about something, you’re pretty vulnerable to have that challenged and I think this is playing with that all the time,” he adds.

As we learn more about Kathleen’s death, we also see different versions of what happens on that fateful night, and Firth and Collette had to play those scenes several times. “It was difficult,” he says. “What really knocked me back is how Toni depicted [the death]. It shook me.”

For Collette, “it’s a strange thing to have to do three times” but inevitably she thinks it was a smart move for the series. “All of these potential ways that [Kathleen] did die…I just think it’s so fantastic that you see all three of them. It was such a clever idea.”

The Staircase, First 3 Episodes Streaming, Thursday, May 5, HBO Max