EXCLUSIVE: 'Unforgettable' Star Simon Kassianides Talks Working With Rosario Dawson and Katherine Heigl

Getty Images

Simon Kassianides is dishing on his role in the upcoming thriller Unforgettable.

The film, which finds the 37-year-old British actor co-starring alongside Rosario Dawson, Katherine Heigl, Geoff Stults and Isabella Rice, centers around a scorned ex-wife's obsession with ruining her former husband's new relationship.

RELATED: Katherine Heigl Has It Out for Rosario Dawson in Stirring 'Unforgettable' First Look

Heigl plays the ex-wife, Tessa Connover, while Dawson portrays her ex-husband's new love, Julia Banks.

Kassianides, who plays the nefarious Michael Vargas, couldn't help but gush about working on a film with "strong" and "powerful women."

"I think that I've been lucky to have worked with two names up there that, you know, when it's a certain level of professionalism, it always comes down to the work and actually always falls back to that theatrical rehearsal space for me," Kassianides tells ET's Jennifer Peros. "Where you’re free to explore ideas, and people are motivated by the nuances of character, and so you’re dealing with intelligent, strong, powerful women."

Despite taking on such a sinister role, Kassianides didn't shy away from interacting with his co-stars once the cameras stopped rolling.

"For me, there was a lot of talking, there was a lot of exploring, and there was a lot of trust, which needed to be established for the character," he explains, adding of his and Dawson's characters: "There's a very human interaction between us, which I think is rooted in that confusion and that heartache."

"I think if I just went in again as a two-dimensional force that wasn't, you know, exploring a past and had that freedom to interact with Rosario as she provided, then I'm not sure the scenes would have worked between us," he admits. "It was wonderful. Like I said, she had that theatrical safe space which allowed us to explore, which is my happiest place as an actor."

EXCLUSIVE: Katherine Heigl Talks 'Super Diverse' Family: 'It's So Much Better Than I Could Have Ever Imagined'

Later in the chat, Kassianides jokingly reveals how Halle Berry helped him prepare for the role.

"I like to think of, you know, Halle Berry or someone I can't have. I'm joking," he says of getting into character. "You have to take the humanity of the character to some point so that it's relatable. I hope that with this character, it's in a place you can relate to that confusion and pain of not having who he loves. But, from a certain point, you have to depart from there and introduce madness."

WATCH: ‘Unforgettable Stars Katherine Heigl and Rosairo Dawson Talk Obsessive Past Relationships

Besides boasting an all-star cast, Unforgettable marks the directorial debut of Denise Di Novi, best known for producing Edward Scissorhands and The Nightmare Before Christmas.

The film, which hits theaters on April 21, was shot before Heigl announced that she and husband, Josh Kelley, were expecting a new addition to their family. Though the 38-year-old actress didn't appear pregnant with son Joshua while shooting, Kassianides was in awe of her on-screen transformation.

"When we were around the set, I was just watching an amazing transformation of this woman," he gushes. "In terms of performing in a space that we have not seen her, and that’s really exciting, to see Katherine Heigl play this person."

MORE: Katherine Heigl Dishes Breastfeeding on the 'Unforgettable' Press Tour and Embarrassing Her Kids