“Are you alone?” the private eye is asked in Roman Polanski’s “Chinatown.” “Isn’t everybody?” he replies. That loneliness is central to a lot of noir heroes, who plunder other people’s secrets while running from their own. The tone was set by Dashiel Hammett, and its greatest practitioner was Raymond Chandler. To observe Humphrey Bogart in Hammett’s “The Maltese Falcon” and Chandler’s “The Big Sleep” (1946) is to see a fundamental type of movie character being born -- a kind of man who occupies human tragedy for a living.

Yet the Bogart character is never merely cold. His detachment masks romanticism, which is why he’s able to idealize bad women. His characters have more education and sensitivity than they need for their line of work. He wrote the rules; later actors were able to slip into the role of noir detective like pulling on a comfortable sweater. But great actors don’t follow rules, they illustrate them. Jack Nicholson’s character J.J. Gittes, who is in every scene of “Chinatown” (1974), takes the Bogart line and gentles it down. He plays a nice, sad man.

We remember the famous bandage plastered on Nicholson’s nose (after the Polanski character slices him), and think of him as a hard-boiled tough guy. Not at all. In one scene he beats a man almost to death, but during his working day he projects a courtly passivity. “I’m in matrimonial work,” he says, and adds, “it’s my metier.” His metier? What’s he doing with a word like that? And why does he answer the telephone so politely, instead of barking “Gittes!” into it? He can be raw, he can tell dirty jokes, he can accuse people of base motives, but all the time there’s a certain detached underlevel that makes his character sympathetic: Like all private eyes, he mud wrestles with pigs, but unlike most of them, he doesn’t like it.