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“Great characters,” says Kyra Sedgwick, who stars in Lionsgate’s  new horror flick The Possession. “If you don’t care about these people or believe these people then you’re really not gonna believe any situational horror that comes up.”

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Co-star Natasha Calis, 13, agreed with her on-screen mother, telling The Hollywood Reporter that reliability and well-developed characters were crucial to big screen terror.

“You can really feel for them and connect with them, and then all of the sudden all this crazy bad stuff happens to them,” she says.

In their film, based on a true story, the broken Brenek family (parents Stephanie (Sedgwick) and Clyde (Jeffrey Dean Morgan) are recently divorced) faces a mysterious demon from a dibbuk box. As 10-year-old Em (Calis) begins displaying bizarre and sinister behavior, the family is forced to seek help from a rabbi’s son (Matisyahu) to exorcise the demon from her body.

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But characters aren’t the only crucial aspect to creating an edge-of-your-seat thriller.

“I think having good music and having a good look to the film can also really help, but I really do think that authenticity saves the day,” adds Sedgwick. “The minute it’s schlocky  or unbelievable or people aren’t really committing to the situation and don’t really believe it themselves, you’re kind of in trouble.”

Her favorite nail-biters?

“The Exorcist and The Omen. Neck and neck,” she says.

For more on the film, including what spooked Morgan on set, click here. The Possession opens Aug. 31.

Email: Sophie.Schillaci@thr.com; Twitter: @SophieSchillaci