Gabriel LaBelle, Rachel Sennott, Willem Dafoe, Dylan O’Brien and J.K. Simmons are among the ensemble in this dramatization of the 90 minutes before the big premiere.

It used to be called a “high concept” movie, back in the days when Saturday Night Live was the hottest thing on television. Make a movie about the origins of that late-night comedy show, but not over a period of weeks or months — just in the 90 minutes before the first episode went on the air in October of 1975. We follow the stresses of the show’s creators and watch the stars practice their sketches while the musicians rehearse and the NBC executives fret. Director Jason Reitman has directed a number of memorable comic films (Thank You For Smoking, Juno, Up in the Air), and we are intrigued to see how he and co-writer Gil Kenan will bring the concept to life.

The film wisely chooses to focus on the show’s creator, Lorne Michaels, played with energy and the right note of befuddlement by Gabriel LaBelle, who played Steven Spielberg’s alter ego in The Fabelmans. The gifted Rachel Sennott plays Rosie Shuster, who was married to Michaels at the time and contributed many of the sketches in the show’s early years. Unfortunately, her part is not developed as sharply as it might be.

Some of the better-known actors in the cast make the strongest impression. Willem Dafoe plays a network executive wary of the appeal of such an irreverent, youth-oriented show, bringing his usual authority and a note of unmistakable wisdom to the kids in the room. One scene in which Michaels and the cast members have to pitch to visiting execs from around the country makes a vivid point about the history of the entertainment business: There is not a single woman in the room. The one woman executive we see is a network censor flummoxed by the show’s sometimes raunchy humor.

J.K. Simmons also has a superb couple of scenes playing Milton Berle, once the king of television comedy, who pays a visit to the SNL set. (Did this really happen? Probably unlikely.) Berle cannot suppress his resentment at the idea of his brand of comedy being supplanted by this group of young upstarts. (There is also a joke about Berle’s well-known physical endowments.) Another character from another generation, Johnny Carson, is also threatened by these newcomers to late-night television, and we hear him in an angry phone call screaming at Michaels for trying to undermine him.

The film is smoothly edited, but there was a problem with the sound at the screening I attended, and some of the dialogue was muffled. In a movie celebrating the fizziness of sketch comedy, you want to hear the zingers. That is only one reason why this film, so fresh and enterprising at many moments, ultimately disappoints.