“I had to start giving him qualities,” he said.

Mr. Eugenides’s method, he said, is to learn a few facts and then use his imagination, working alone in his large Tudor-style house in Princeton, N.J., a short walk from the university, where he teaches part time.

“I wanted a character who was impressive in college for a range of knowledge,” he said. “I toyed with making him a physicist or chemist,” he said, but he settled on biology because that was a subject he had looked into when writing a previous novel, “Middlesex” (2002), about a person whose sex is ambiguous.

As part of the “best and worst boyfriend” package, he decided to give Leonard bipolar disease. Mr. Eugenides said he had never known anyone with the disorder, but looked it up online, finding symptoms and drugs and side effects. He asked himself what it might feel like to be manic, to be depressed.

“Maybe there was a time when I did youthful, risky things,” Mr. Eugenides said. “I stayed up all night. There were rare moments when I felt my most brilliant and my most conceited.” What might it feel like to be in that state for months at a time? What might Leonard say? What would he do?

He asked similar questions about Leonard’s science.

“What would he be doing in 1982? What science was current at that time?” He went to the Internet and stumbled upon studies of yeast mating systems. Perfect, he thought. It fit well with the theme of his book.

He asked his wife, an artist who spent a winter in Cold Spring Harbor, but not at the research center, what she remembered about it. He found, read and somehow understood Dr. Klar’s paper.