Robert Downey Jr claims his time spent in rehab helped him during the filming of new movie ZODIAC - because it was like working on a forced labour camp. Seven filmmaker David Fincher directed the movie and is said to have made the cast - including Downey Jr and his co-star Jake Gyllenhaal - perform up to 90 takes to get certain scenes right. Downey says, "I just decided, aside from several times I wanted to garrotte him, that I was going to give him what he wanted. "I think I'm the perfect person to work for him, because I understand gulags." Zodiac is based on two books by Robert Graysmith about the Zodiac Killer, who reined terror in the San Francisco Bay Area during the 1960s and 1970s.