A Mighty Wind is often called a mockumentary and has been compared to serious documentaries such as The Weavers: Wasn't That A Time (which I have not seen). But director Christopher Guest hasn't tried very hard to make something that looks like a documentary. The movie drifts uneasily between documentary and dramatic modes without establishing a clear stylistic difference between the two. This may be one of the reasons that it reminds me of Robert Altman's Nashville more than it does a music documentary. Although Nashville has ostensibly more serious aspirations, much of the fun of watching it comes from its caricatures of musicians who take themselves more seriously than we do, and the same is true of A Mighty Wind. Nashville tries to catch the musicians on the rise or at their peaks, whereas A Mighty Wind picks them up well after their primes. Both movies juggle large casts of actors who write and perform original songs that mimic the genre, with varying degrees of accuracy, and both movies build to concerts with dramatic conclusions.
Guest arguably shows more compassion for his central characters than does Altman, but ultimately it's Nashville that feels more real, despite the fact that it rarely poses as a documentary. Altman and his collaborators capture daily life with close observations, watching Lily Tomlin and Ned Beatty interact with their kids at home, say. Guest is primarily focused on making us laugh and detours only occasionally, but affectionately, such as when Michael McKean hears Eugene Levy and Catherine O'Hara singing what he calls "that pretty song" on a backstage monitor and unironically asks Harry Shearer to turn up the volume. It furthers the plot — Guest wants all of the characters to witness the climax from the wings and needs to bring them together — but he could have accomplished the same thing with meanness. The mockumentarian is unexpectedly sweet this time out. The music, too, is silly but understated enough that in the right context it could probably be taken seriously. The main reason to see the movie is to watch the great cast having fun, although its size tends to squeeze the performances down to snippets.