Errol Morris poses during a portrait session during the 70th Venice International Film Festival at the Quattro Fontane on September 5, 2013 in Venice, Italy. (Ian Gavan/Getty Images)

Errol Morris, whose documentary about former defense secretary Donald H. Rumsfeld, “The Unknown Known,” opens in theaters and on demand Friday, encountered some unexpected reactions — call them his own unknown knowns — in the course of making and presenting the film, which had a three-part festival circuit debut last year in Telluride, Venice and Toronto.

“I made a list of all the criticisms I expected to hear about this movie, and I’m hearing ’em,” Morris said over an egg-white omelet and tea at the Toronto International Film Festival last September. “And I’m hearing — which is always one of the great pleasures — criticisms that I never even anticipated.

“Someone asked me, ‘How come you made this movie so soon after the events?’ In ‘The Fog of War’ [former defense secretary Robert S.] McNamara leaves the Defense Department in 1967 and over 30 years pass before I put him in front of a camera. Rumsfeld left in 2006 so only, say, six years pass by. I pointed out one salient detail, that Robert McNamara, when I first interviewed him, was 86 years old . . . and he was in failing health. Donald Rumsfeld had just turned 81. If I waited another, say, 30 years, chances are that he would not have been available to be interviewed.”

Morris, 66, is a tall, imposing figure, given to rambling, complex replies to even the simplest questions, inserting long pauses between sentences that lend everything he says an academic, philosophical air. It’s a conversational style very much in keeping with Morris’s own films — including the groundbreaking 1988 nonfiction crime thriller “The Thin Blue Line,” the alternately delightful and somewhat creepy “Fast, Cheap and Out of Control” and the aforementioned “The Fog of War,” his 2003 portrait of McNamara that earned him his first Oscar — all of which share a similar ruminative sensibility and thematic interest in the self-justifying narratives of their subjects.

But Morris’s love of talking and thinking and thinking while talking is also a well-worn strategy of public figures who are used to being interviewed — to run out the clock, keep their interlocutors on the back foot and set the pace according to their terms. There are times in talking with Morris — observing firsthand his intellectual curiosity, love of verbal precision and ability to control the table — that it feels almost like talking with that master of the form: Rumsfeld himself.

“The Unknown Known” stands as an engaging but ultimately confounding testament to Rumsfeld’s well-known relationship with language, which became public knowledge during the Iraq War in 2003 when his news conferences became tutorials in the definitions of “guerilla warfare” and “terrorism.” During his tenure at the Pentagon, he sent thousands of memos, nicknamed “snowflakes,” engaging colleagues and underlings in elegant etymological inquiries about words and their meanings, occasionally sending them to the Oxford English Dictionary for backup.

That rhetorical style is in full force in “The Unknown Known” (the title is play on one of Rumsfeld’s best-known loop-de-loop answers to a reporter’s question), in which the genial, expansively charismatic Rumsfeld answers Morris’s questions with a series of verbal feints, evasions and traps — each seemingly more deep but utterly obfuscatory than the last.

Which brings us to the criticisms Morris was bracing for back in September. “What I did anticipate was, ‘Your questions are not hard-hitting enough, there’s nothing new here, it’s just simply a rehash of material that everybody already knows,” Morris said. “But the idea is to tell the story of history from the inside out.”

It’s true that, for partisans who can never forgive Rumsfeld his part in the run-up and execution of the Iraq War, “The Unknown Known” can be a maddening experience: Although Morris occasionally confronts his subject about contradictions or outright misstatements, he just as often lets Rumsfeld have his say, unchallenged. “I think the entire movie is a confrontation,” Morris insisted. “Many, many, many successive confrontations.”

As an example, the filmmaker points to a passage in the film when Rumsfeld insists that the majority of Americans never believed that Iraqi leader Saddam Hussein had anything to do with the events of Sept. 11, 2001 (and, by extension, that he and his Bush administration colleagues were not responsible for such an association).

In the film, Morris quotes a 2003 Washington Post poll showing that 69 percent of Americans believed Hussein was involved in 9/11, then cuts to Rumsfeld suggesting the same at a news conference when he sarcastically rejects suggestions to the contrary. “It isn’t a confrontation in the sense of [me] saying, ‘You’re wrong,’ ” Morris said. “But, golly gee whiz, it’s all there.”

If Morris’s oblique strategy invites frustration, so does Rumsfeld’s seeming inability or unwillingness to confront the implications of his policies and actions, whether they have to do with interrogation practices at Guantanamo Bay or the planning of the war itself. Whereas “The Fog of War” presented a fascinating portrait of McNamara as a historical figure reflecting, often painfully, on the events he witnessed or authored, in “The Unknown Known,” Rumsfeld often offers vague, inconclusive cliches: About Vietnam, he says simply, “Some things work out, some things don’t. That didn’t.” About Iraq, “Time will tell.”

At one point, Rumsfeld insists that “enhanced” interrogation techniques did not migrate from Guantanamo to Abu Ghraib (whence came the notoriously damning photos of U.S. soldiers mistreating Iraqi prisoners); Morris then presents him with the findings of an independent review panel chaired by former defense secretary James R. Schlesinger, whose report concluded that Pentagon policies led to “uncertainties” in the field and that “the augmented techniques” that Rumsfeld signed off on for Guantanamo “migrated to Afghanistan and Iraq, where they were neither limited nor safeguarded.” Rumsfeld nods and replies, “I think that’s a fair assessment.” After a silent stare-down with Morris, Rumsfeld finally concludes that, in every war, “things occur that shouldn’t occur.”

For Morris, such verbal shrugs may be artful dodges, but they’re also revelatory of Rumsfeld’s moral imagination — or, the filmmaker is quick to add, the lack thereof. “You’re left with a strange anxiety about him,” Morris said, adding that he would welcome the chance to interview Rumsfeld again. “I suppose if I was Mike Wallace or David Frost or whoever, I’d back [him] into a corner. But I love those moments, because I don't even know where I am anymore. I don’t know whether he’s in any way self-aware, whether he is lying, whether he’s just in some strange alternate universe, the Rumsfeld universe. . . . There’s a ‘j’accuse’ there, but it’s my ‘j’accuse’.”

Having now made psychological portraits of two consummate Washington operators, has Morris reached any new conclusions about how political power works? “We have a picture of our democracy [that] may be an illusion,” the filmmaker said. “I never had much patience with the idea that it’s all economic, or that it’s all power politics. I think I come closer to how I feel when I ask Rumsfeld about Shakespeare. To me, the world is Shakespearean. Rumsfeld is constantly denying that anybody in government acts because of jealousy, greed, ambition, hatred. . . . I think history is crazy. I think we are crazy. We create this veneer of seeming rationality over the craziness. But one thing that I think is clear in this movie is that the craziness keeps poking through.”

The Unknown Known

PG-13. At Angelika Film Center Mosaic and Avalon. Contains some disturbing material and brief nudity. 103 minutes.