"He hates the idea of the film and asked me not to do it," Cumberbatch told NYMag.
The British actor found the request odd since he holds Assange in high esteem.
"No matter how you cut it, he's done us a massive service, to wake us up to the zombielike way we absorb our news."
Instead, Cumberbatch — who has never formally met Assange — pointed out that had he not taken the role, someone else would have swooped it up.
"Wouldn't you rather it's someone who has your ear, who could steer the film to a place that's more accurate or balanced?"
"The Fifth Estate" is directed by Bill Condon and will follow the early days of the news-leaking site.
The DreamWorks' film was just pushed up to an October 11 release date.
"The Fifth Estate" was originally slated for a November release.
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