D. T. Max first contributed to the New Yorker in 1997 and has been a staff writer since 2010. He is the author of “The Family That Couldn’t Sleep: A Medical Mystery,” a cultural and scientific study of fatal familial insomnia disease; “Every Love Story Is a Ghost Story: A Life of David Foster Wallace,” and the topic of our conversation, “Finale: Late Conversations with Stephen Sondheim.” He has been the books editor of the New York Observer, a writer for the Times Magazine, and a pseudonymous food reviewer for Paper. He contributed the afterword to the New York Review Books Classics reissue of William McPherson’s 1984 novel, “Testing the Current.”