Lifelong Oscar bridesmaid Leonardo DiCaprio is moving ahead with The Crowded Room, an extremely buzzy biopic in which he’ll play Billy Milligan, the first person to successfully use multiple personality disorder as a legal defense.

Milligan, who died in December of 2014, was charged with robbery and raping three women on the Ohio State University campus in the 1970s. While preparing his defense, Milligan was diagnosed with multiple personality disorder. He and his lawyers pleaded insanity, contending that two of Milligan’s alleged 24 personalites committed the crimes without his knowledge. He was the first to use this defense and be acquitted because of it.

Some of the man’s personalities included Adalana, a lesbian who admitted to the rapes; Ragen, a Yugoslavian communist who admitted to the robberies; and Arthur, a high-strung but proper Englishman. Perhaps the most fascinating part of Milligan’s story is that Hollywood has not yet...