Oscar winner Natalie Portman explains how her Knight of Cups costar Christian Bale kept her guessing during filming
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On the set of her new film Knight of Cups, Natalie Portman says she and costar Christian Bale were often encouraged to go rogue with their dialogue and actions – which the star says was both incredibly freeing and fun, but also sometimes frightening.

“Christian would do surprising things all the time,” Portman, 34, tells PEOPLE of her first time working with Bale in Terrence Malick’s philosophical drama Knight of Cups. “It’s fun to watch – and sometimes scary, like when he’s like diving off a dock into the ocean when you don’t expect it in the middle of a scene. But he also keeps everything feeling very alive and spontaneous. It was really a fun, unusual experience.”

The film revolves around handsome lost soul Rick (Bale), who is on a journey of personal discovery, much of which he experiences via romantic relationships with women, past, present and future. Portman plays Elizabeth, a well-meaning young woman who strays from her husband and becomes entangled in a passionate affair with Rick.

“It’s got that complication to it,” Portman says of what drew her to the role. “Because in one sense [their relationship] has the most authentic love, but also it’s the most forbidden. So there’s this gentleness and a generosity between them, but it’s the wrong relationship to be having.”

Like her work with Bale, Portman describes director Terrence Malick as an inspiring, creative force of nature.

“He really doesn’t go along with any rules and rituals of filmmaking, ” she says. “Every movie usually follows a very similar schedule and setup, and he frees himself from all of that. He never tells you what to think or feel, which I really appreciate. ”

The film’s ensemble cast also includes Cate Blanchett, Brian Dennehy, Frieda Pinto, Imogen Poots, Wes Bentley, Teresa Palmer, Isabel Lucas and Antonio Banderas.

Knight of Cups is playing in select theaters.