Tatum told E! that working with the actress, best known for her Oscar-nominated role in The Girl with the Dragon Tattoo, was a slightly nerve-wracking experience.
He said: "She's the most mousy and terrifying person I've ever met. She's such a little introvert."
Despite her shy nature, the actor also revealed that she was still a pleasure to act with. "She's also the sweetest little thing you've ever met," he added.
In The Bitter Pill, Mara plays a young wife suffering from depression when her husband returns home after a four-year stint in prison for a white-collar crime.
"She's truly one of the most fabulous actresses," Tatum said of his co-star. "She has a skill set that I have never seen with the actresses I've worked with. Everybody's different and have the things that they're strong at, but she's special, for sure."