The 50th anniversary of the assassination of John F. Kennedy will be memorialized by Jessica Chastain,Viola Davis, Channing Tatum, Anne Hathaway andMichelle Williams in the documentary “Letters to Jackie,” in which 20 of more than 800,000 condolence letters sent to his widow are read. The actors will join director Bill Couturié and presenter Christy Scott Cashman at AFI’s premiere at the Kennedy Presidential Library in Boston on Sept. 17. Other letter readers include Betty White and Zooey Deschanel.