Sadie Sink explored the dark side of her character in 'The Whale'.

The 20-year-old actress plays the role of Ellie in Darren Aronofsky's Oscar-tipped film and says she was unable to decide if her alter ego was good or evil as she builds a relationship with her estranged father Charlie (Brendan Fraser).

When asked about her character's intentions, Sadie told The Hollywood Reporter: "I don't know, I think when I was filming, definitely for the earlier scenes, I had moments like where I was, 'Oh there's some real, real darkness in her.' You know there is, but I think it's just developed over time coming from this great deal of pain that she's in.

"And I think she has no idea who she is, she's very insecure, I think."

Sadie discussed the complex relationship between Ellie and Charlie and thinks that her character plays a "game" with her obese father in the film.

The 'Stranger Things' star said: "It's this game that she's become really good at playing that's kind of a challenge for Charlie, but the thing about him is that he's able to kind of see past all of it and I don't want to ensure her that there's a part of herself that he sees so clearly that I think is good, I think there's good in her."

Meanwhile, Sadie's performance in the movie impressed her co-star Brendan so much that he kept forgetting his lines.

The 54-year-old actor said: "The fluidity that Sadie plays, this beautiful, controlled rage, was breathtaking to me.

"I was constantly going up on my lines. And that's just because I have cobwebs upstairs and I was wearing a lot of gear, but also because I was just slack-jawed. I'm not joking.

"Watching her work — she won the game ball every day we worked, I'm telling you."