In his first movie as a director, Jon Stewart has made things easy on himself, but he’s also made things hard. The easy part is that he has made a movie that mostly involves just two people in a room talking. The hard part is that it’s very difficult to make that kind of movie interesting.

"Rosewater" is based on the prison memoir of Maziar Bahari, an Iranian-Canadian journalist who got into trouble, not only for filming government murders on the streets of Tehran (which he did), but for something completely ridiculous: While on assignment in Iran for Newsweek, he gave an interview to Jason Jones of Stewart’s "The Daily Show," who took on the comic persona of an American spy. Upon seeing this, the Iranian authorities believed Jones was a real spy and that Bahari was conspiring with him — as though real spies have TV shows in America.

We soon realize that Bahari’s inquisitors are profoundly stupid and that their stupidity makes them dangerous. They seem to genuinely believe that the western democracies dictate editorial decisions at magazines and newspapers, a misconception that leads them to conclude that all reporters are spies. The material is ripe for black comedy, but Stewart’s screenplay, staying true to Bahari’s real-life experience, steers a middle course. It’s sometimes scary, sometimes funny and sometimes absurd, but never any of those things fully, or effectively.

‘ROSEWATER’ Rated: R Opens Friday at Dole Cannery Stadium 18 and Kahala 8

The early scenes are the best, with Stewart deftly mixing actual footage from the 2009 Iranian street demonstrations with his own recreations. But once Bahari goes to prison, so does the movie. Prison is boring, whether you’re in it and staring at the four walls — or on the outside watching a guy staring at the four walls.

The problem here is one of basic structure: Bahari’s experience just didn’t have a dramatic arc. Once he was imprisoned, his story pretty much flatlined. Stewart tries to compensate by offering the illusion of movement, the idea being that Bahari is gradually evolving over the course of his ordeal. But Stewart doesn’t have the finesse to pull off that sleight of hand. The inescapable truth is that even if Bahari grew as a person, his path to freedom wasn’t an internal matter but had everything to do with things occurring in the external world, such as protests by the international community.

Still, the story feels important, and the subject is worthy, and those things help. Bahari was fulfilling his function as a journalist, and he was punished for it, just as others are still being punished for doing the same, all over the world. Stewart’s accompanying idea, that the ubiquity of cellphones and the Internet make it harder for tyrants to work in darkness, also has impact — as a concept.

But "Rosewater" is not just a set of nice ideas. It’s a drama, and as a drama it has a promising opening, a reasonably interesting conclusion and a dead middle. To fill that middle, Stewart gives us flashbacks, voiceovers, and visions of lost family members. He gives us interrogation scenes that are, at first, ominous, then monotonous. He tries a variety of tones without ever quite committing, as though he didn’t realize that the movie’s impact depended on his sharpening its focus, not widening it.

Gael Garcia Bernal is an intelligent and sympathetic focus. If you’re going to show a guy getting punched around and terrorized, it’s best to cast someone the audience automatically likes. Danish actor Kim Bodnia plays his chief tormenter, a menacing figure who becomes increasingly pathetic as the film wears on, not in a farcical way, but in a way that makes us wonder if any human could really be that stupid. Maybe it’s possible, but it feels a little forced.

Still, for a first effort, Stewart hasn’t done a bad job. There’s just the nagging feeling with "Rosewater" that a better movie was possible.