Sherlock's Benedict Cumberbatch has already signed on to play WikiLeaks founder Julian Assange.
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Variety reports that Stevens, who plays Matthew Crawley in Downton Abbey, will play a hacker close to Daniel Domscheit-Berg, one of the company's most prominent spokesmen.
Last week, it was reported that Stevens won't return full-time to the ITV drama for its fourth series.
Domscheit-Berg will be played by Daniel Bruhl, after James McAvoy had been linked with the role. Swedish actress Alicia Vikander (Anna Karenina, A Royal Affair) will also fill out the cast.
The film is as-yet officially untitled, but has reportedly been given the working title of The Man Who Sold the World as production heads towards a January start date.
Cumberbatch will next be seen in the upcoming sci-fi sequel Star Trek Into Darkness, and is also set to play Brian Epstein in a new Beatles biopic helmed by Sherlock director Paul McGuigan.
Watch Benedict Cumberbatch discuss Sherlock with Digital Spy below:
