The other Monday evening, three men in their forties were hanging out on the mezzanine of the Hudson Theatre, in midtown. They were John Mulaney, the comedian; Alex Timbers, the Broadway director; and Simon Rich, the writer. The three—two quite tall, one a bit shorter, all about equally floppy-haired—had gathered for the night’s performance of “All In,” a limited-run “comedy about love,” which Rich wrote, Timbers directed, and Mulaney stars in. Timbers, in an olive-green button-down, was examining the pictures on the wall. “This was when Elvis went on ‘The Steve Allen Show,’ and Allen humiliated him by making him sing ‘Hound Dog’ to a dog,” he said, of a large framed photograph of a mournful-looking young Presley standing beside an even more mournful-looking basset wearing a small top hat.…