Jane Fonda was originally cast as Julia. However, when the producers had trouble casting the role of Lillian Hellman, they decided to recast Fonda in the lead.
Jack Nicholson was offered the role of Dashiell Hammett, and was actually keen to take it. Fred Zinnemann later retracted the offer, however, when he felt put off by the amount of time it was taking Nicholson to read the script - just under a week.
Early on, director Fred Zinnemann actually considered casting Meryl Streep in the title role. However, as Streep was almost totally unknown as an actress - she had only one play to her credit, and never appeared in a film - Zinnemann decided to cast Vanessa Redgrave instead.
During the casting process, both Jane Fonda and Vanessa Redgrave's names were mentioned as possible stars for the film. The producers initially vetoed both actresses on the advice of the publicity department, fearing that the absolute worst option would be to cast Fonda and Redgrave, both of whom were known for their outspoken political beliefs, in a film together. In the end, of course, both actresses were cast and the film went on to great critical and box office success.
It was during her Best Supporting Actress Oscar acceptance speech for this movie that Vanessa Redgrave incurred the wrath of the Jewish Defense League (JDL) when she declared: "... and I salute you, and I pay tribute to you, and I think you should be very proud, that in the last few weeks you have stood firm, and you have refused to be intimidated by the threats of a small bunch of Zionist hoodlums, whose behavior is an insult to the stature of Jews all over the world, and to their great and heroic record of struggle against fascism and oppression. And I pledge to you that I will continue to fight against anti-Semitism and fascism. Thank you." (Dorothy Chandler Pavilion - 3 April 1978)