Bret Easton Ellis looked uneasy. He was sitting in the Schoenfeld Theater, waiting to see the Broadway musical based on his controversial novel “American Psycho” for the first time. He shifted in his seat and looked around at the crowd, then leaned back and sighed.

“This is just another surreal aspect of my life,” he said. “It feels like the weirdest dream. I don’t know,” he continued wearily, his voice trailing off.

Mr. Ellis, who had recently flown in from Los Angeles, seemed to be having doubts about the whole endeavor. Before heading to the performance, he had a panicked conversation with his boyfriend. “I said, ‘God, what the hell are they going to do with this?’”

It’s a fair question. How do you turn a wildly transgressive, experimental novel about a slick Wall Street investment banker-turned serial killer into a mainstream Broadway musical?