One of Hayden Panettiere's costars is sharing some insight into how she was allegedly feeling on set of what would turn out to be her final film.

Justin Chatwin spoke with People on Monday (August 17) about the sudden death of his Sleepwalker costar at 36 and how he remembers bonding with Panettiere behind the scenes despite the difficult subject matter of the movie, which follows a woman struggling with the loss of her daughter from a car crash. The pair also previously worked together on the TV show Heroes.

"We hit it off great and it was an awesome shoot, but it was about domestic abuse and it was subject matter that may have hit close to home, because she did mention it," he said, adding, "I could tell she was struggling, and I was like, 'I don't think it's this movie.'"

He continued, "There's a handful of women that I've worked with that I really liked, and I really loved working with her, but I also know that she was really lonely."

Chatwin said that Panettiere "opened up to [him] about a lot of things," including her "struggles with addiction" and "trauma." Chatwin, who shared he has battled his own addiction struggles and has been sober for 15 years, recalled how "sweet" the late actress was to talk to.

"I'm sure that everyone else is going to say, 'Oh, she's amazing. She's such a sweet person on set,' because she was," he said. "But we're trained to be actors, so we're acting on set, you know? And she is sweet. She had a beautiful heart. But we're trained to be on, and I really like spending time with her because she took that off, and she was authentic with me about her life and she opened up about her struggles with addiction. And who knows where it comes from? Does it come from Hollywood? Does it come from home? And a lot of times, it's both."

Panettiere died August 16 at 36 years old after she was found unresponsive at a residence in Greenville, South Carolina. According to People, officials were heard discussing discussing an "overdose" and "cardiac arrest" around the time workers responded to the scene. The Greenville County Coroner's Office later said that responders arrived to find a female in cardiac arrest, later identified as Panettiere, and despite resuscitation efforts, she was pronounced dead at the scene.

Following news of her death, tributes from friends and colleagues poured out on social media mourning the heartbreaking loss, including her ex-fiancé Wladimir Klitschko.