CAMBRIDGE — Errol Morris is no stranger to controversy. Most of the documentaries that make up his career have thrived on it. “The Thin Blue Line’’ (1988) challenged a corrupt justice system in Texas; “Mr. Death’’ (1999) got up close and personal with Holocaust denier and execution-device designer Fred Leuchter, Jr.; “The Fog of War’’ (2003), which earned him an Oscar, largely consisted of a sit-down discussion with Vietnam War-era Secretary of Defense Robert McNamara; “Standard Operating Procedure’’ (2008) looked at what US forces were doing to detainees at Abu Ghraib Prison.

But Morris hasn’t focused only on the serious side of things. There was a sense of awe floating through “A Brief History of Time (1991), his take on the life of Stephen Hawking, and feelings of both wonder and whimsy in the four disparate characters and chapters of “Fast, Cheap & Out of Control’’ (1997).

With “The B-Side: Elsa Dorfman’s Portrait Photography,’’ which opens July 14, Morris opts for an insightful, inspiring, entertaining chat with a subject, who’s a longtime friend, fellow Cambridge resident Dorfman, who pioneered the use of the large-format Polaroid 20x24 camera to make portraits of both famous and everyday people.

Sitting in his airy and comfortably cluttered East Cambridge office, where the head and neck, flowing mane intact, of a taxidermied horse is mounted on a wall, and books — a couple of Napoleon biographies, “The Collapse: The Accidental Opening of the Berlin Wall,’’ and one simply titled “Cannibalism,’’ among them — are piled on his large wooden desk, Morris, 69, talked about his relationship with Dorfman, the genesis of the film, and how it differs from his earlier projects.

Q. When did you first meet Elsa Dorfman?

A. My wife met her before I did. She arranged to have Elsa take our son’s picture when he was 4 or 5 years old. That was a long time ago; he just turned 30. And we slowly became friends with Elsa over the years.

Q. When, how, and why did you decide on her as a subject?

A. We were in the middle of a Netflix project called “Wormwood’’ [about Cold War military scientist Frank Olson]. We had a little bit of time before we started shooting, so we decided that we would shoot something else. I had had Elsa on my mind for years. I’d spent an afternoon with her in her garage. She would open the drawers of various flat files and she would show me Polaroids. She would show me her collection of B-side photographs and she would start telling stories. And I thought, “That’s a movie! Just her photographs and her stories about the photographs, that’s enough.’’

Q. The term “B-side’’ is well known for being the other song on a 45 rpm record, the one that wasn’t the hit. How does that relate to your film?

A. I first heard about it, as applied to her photographs, from Elsa. The entire collection of Polaroids in those flat files were B-side photographs. By B-side she meant that she took two photographs, and she gave the family the choice of one, that they would pay for, and she would keep the rejects. The rejects then became known as the B-sides. So it’s a collection of photographic rejects. The irony is that the rejects are often better than the photographs that were chosen.

Q. Was Elsa willing to do the film as soon as you mentioned it to her, or was there some trepidation?

A. I don’t believe there was any trepidation on her part. Perhaps a kind of disbelief that I would follow through and do it, but not an unwillingness. When we actually decided to do it, there was a deadline. She was moving these giant Polaroids out of her stairwell. Gentle Giant [movers] was scheduled to arrive on such and such a date, and they were going to take them out of the house and put them in storage. We had a date and we decided that we should record it, that it would be a part of the film. So we just started, and we did the whole thing very quickly. We were done shooting in six days.

Q. Did she catch you off guard by anything she said in the film?

A. Elsa constantly caught me off guard! But not intentionally. Part of doing something like this is being quick on your feet. You’re constantly making decisions because it’s not scripted. I don’t know what they’re going to say; and in this case I didn’t know what she was going to do. One thing Elsa did very early on, that I have come to love, is she would take Polaroid photographs out of the flat file and hold them up in front of her. But the photographs are so big, 20-by-24, that she would obscure herself. So there was a challenge of how you shoot something like this. So what I did was I showed parts of the photographs, but they became part- Elsa displaying the photograph. She was holding them up and sometimes you just see her eyes peeking over the top of them, sometimes you just see her hands on the edge of them, and she’s still talking.

Q. You’ve used your teleprompter-like device, the Interrotron, in all of your films since “Fast, Cheap & Out of Control,’’ which creates an intimate atmosphere with your subject but gave the films something of a static feeling. “The B-Side’’ has cameras all over the place. What happened here?

A. In shooting “Wormwood,’’ I decided, instead of using my Interrotron, to use multiple cameras. I often used as many as 10 cameras on the show, and I had up to five cameras in the Elsa film.

Q. Did that prove to be a new freedom for you?

A. It’s something different, so in that sense it’s a new freedom. Then I decided to use this lens system called Revolution, that allows you to move the camera into odd positions, and it gives you an enormous amount of flexibility. So I was able to have a conversation with Elsa, and at the same time I could operate one of the cameras, which is something that I have never done before. I abandoned the whole idea of having her look directly into the lens. Instead you have a whole variety of angles, and I really like the way it looks.

Ed Symkus can be reached at esymkus@rcn.com.