Stewart, who is dating Pattinson and attended the Cannes premiere of his new movie, told MTV News that she was very impressed by his performance.
"He's so good in it!" Stewart said. "He's really, really [good]."
She continued: "I don't even know how he [did it]. I couldn't even understand it. It's so good, it's so cool, I'm so proud of him. Thank God!"
Stewart recently revealed that she would love to work with Pattinson again, saying that she is "dying to make another movie" with him and adding that they are a "good team".
Pattinson has confirmed that he will work on another movie with Cosmopolis director David Cronenberg in the future.
> Cosmopolis review: Robert Pattinson hits Cannes Film Festival 2012
Watch a trailer for Cosmopolis below:
Photo gallery - Cannes premiere of Cosmopolis:
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