The dark comedy will begin shooting early next year. Heller will direct from a script by Nicole Holofcener.
Producers are Anne Carey and Amy Nauiokas of Archer Gray, partnered with Israel's friend and longtime confidant David Yarnell. Jawal Nga executive produces, reported Variety.
McCarthy will portray Israel, who made her living in the 1970s and '80s writing profiles of stars such as Katharine Hepburn and Tallulah Bankhead, cosmetics executive Estee Lauder, and journalist Dorothy Kilgallen.
When the forgeries started to raise suspicion, she turned to stealing the actual letters from library archives and sold them. In 1993, Israel pleaded guilty to conspiracy to transport stolen property and served six months under house arrest. She died in 2014.
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Julianne Moore was initially set to star in the film, but exited last year due to creative differences. At that point, Holofcener had been attached to direct, but she is no longer available.
Matthew Greenfield, Searchlight's new co-head of production (with David Greenbaum) is shepherding the project.
McCarthy starred in "The Boss" and will be seen next in "Ghostbusters." Heller has directed episodes of "Casual" and "Transparent.