Errol Morris recalls that when he first met Donald H. Rumsfeld, he asked that former defense secretary for President George W. Bush what he thought of “The Fog of War,” Mr. Morris’s 2003 Oscar-winning documentary about Robert S. McNamara, another towering Pentagon chief toppled by a misguided war.

Mr. Rumsfeld said that he disliked it because McNamara, who spent much of the film regretting past mistakes, had nothing to apologize for.

In short, a film about this man wasn’t likely to reveal any deep, dark secrets. Instead, Mr. Morris decided to focus on the memos that Mr. Rumsfeld wrote — “snowflakes,” they were called, because he blanketed his staff with thousands of them, like blizzards of white paper — as a Rosetta stone to his subject’s character.

The resulting film, “The Unknown Known,” which opens on Friday, is structured much like “The Fog of War.” For 96 minutes (cut down from 34 hours of conversation), Mr. Rumsfeld looks into the camera, answering, or finessing, questions about Iraq, his work for several presidents, the nature of truth and more.

Throughout the dialogue, Mr. Morris uses the memos as the opening wedges of inquiry. Several times, his camera pans vast stacks of file cabinets, while Danny Elfman’s score (like Philip Glass’s for “The Fog of War” but less minimalist, more celestial) cues a choir to chant ominously.

The goal, Mr. Morris said in a phone interview in late November, when the film was on the festival circuit, was “to tell history from the inside out, not from the outside in,” to explore “how Donald Rumsfeld sees himself and accounts for himself.”

Yet this technique has its limits when confronting a figure so practiced in evasion and so averse to introspection.

McNamara was 85 when “The Fog of War” was made, but he was still sharp, more than three decades beyond his own public disgrace, and eager to recite the lessons he’d since learned. He was deceptive, or at best forgetful, about the Vietnam War and the Cuban missile crisis, recounting his role as more dovish than the archives indicate. But the film was a fascinating, almost tragic portrait of a man still lost in the fog, grappling with his legacy.

No one could expect Mr. Rumsfeld, now 81, to engage in self-criticism, certainly not in front of a camera. Still, some of his replies are so vapid that it’s hard to tell whether he’s slippery or shallow.

Asked whether invading Iraq was a mistake, Mr. Rumsfeld plaintively says, “I guess time will tell.”