Actress Hayden Panettiere was experiencing significant struggles and feelings of loneliness while filming her last movie, "Sleepwalker," according to co-star Justin Chatwin. Panettiere died recently, just before her 37th birthday, from a suspected overdose.

In the psychological thriller "Sleepwalker," Panettiere portrayed a mother grappling with the loss of her daughter in a car accident. The character also endured domestic abuse, a theme that resonated with Panettiere's own experiences with her on-off boyfriend, Brian Hickerson, who has admitted to being abusive.

"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," Chatwin, who played Panettiere's abusive husband, told People. He noted that Panettiere, whom he had known previously, "seemed different" during the filming of what would become her final film.

Chatwin observed that Panettiere was "struggling" and "really lonely." He added, "I could tell she was struggling, and I was like, ‘I don’t think it’s this movie.’" He also shared, "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."

During the production of "Sleepwalker," which was released in January, Panettiere was open about her challenges. "She was authentic… about her life and she opened up about her struggles with addiction," Chatwin said. He described her as "delightful and super open" and expressed sadness over her passing.

Panettiere's death occurred just months after the release of her memoir, "This Is Me: A Reckoning." In the book, she candidly discussed her battles with alcohol and opioid addiction, postpartum depression, domestic abuse, and the pressures of growing up in the spotlight.

Brandon Auman, the writer and director of "Sleepwalker," remembered Panettiere as a "bright, open, [and] numinous spirit." He told People that she was honest about how her life experiences related to her character, describing her as "complex and deeply introspective."

Panettiere, who had a daughter named Kyla with ex-boxing champion Wladimir Klitschko, was found unresponsive on Sunday in a Greenville, South Carolina, apartment. Authorities reported she suffered cardiac arrest following a suspected overdose. She had traveled from Los Angeles with Hickerson the day before, though it is unclear if he was the one who called for help.