EXCLUSIVE: Thomas Mann and Jenny Slate are in talks to join Dakota Fanning in Brain on Fire. Glassland's Gerard Barrett is directing his scripted adaptation of the Susannah Cahalan's bestselling memoir Brain on Fire: My Month of Madness, with Fanning playing the author during a terrifying and sudden descent into insanity as the result of a mysterious illness. Charlize Theron, Beth Kono, and AJ Dix are producing for Denver and Delilah Films, and Rob Merilees for Foundation…