“Studio 666” is a horror comedy about the fun and frolic that ensues whenFoo Fightersfrontman Dave Grohl becomes possessed by the devil.

That this movie actually works, in a vague sort of way, is largely thanks to Grohl’s personality. Put another rock star in there and it might not have been funny. In fact, it might have beenplausible.But Grohl, who is about as good-natured as a sheepdog, is the last rocker you’d expect to become demon possessed.

The movie, based on a story by Grohl, finds the Foo Fighters needing to record a 10th album, but Grohl has no ideas. Taking inspiration from Led Zeppelin, who once recorded in a haunted house, the band decides to live in a big house in Encino (Los Angeles County) and devise an album while on the premises.

Some of the best things in “Studio 666” have to do with being in a band and working as an artist in the hard rock medium. At one point, Grohl devises some guitar riffs upon which to base songs, but he’s blocked and keeps coming up with riffs for songs that he has already written.

In a funny moment that is also revealing, he describes a song that he has in his head, telling his bandmates what sounds they should be making on their respective instruments. It’s a comic tour de force, with Grohl waving his arms and tossing his hair. But at the same time, the moment seems grounded in an accurate description of what a rock composer might hear in his head as he writes this kind of song.

In the basement of the house, Grohl discovers a reel-to-reel tape recorder, with an unfinished song that he believes might give him his entire next album. He becomes possessed by the music and for several days on end, he puts together a song that he can’t seem to end, that goes on 40 minutes — no lyrics, just various riffs. The funny thing is that it actually sounds pretty good.

So “Studio 666” is OK musically, and is reasonably funny, benefiting from Grohl, who is a natural comedian. But the movie drags because of its length. Too often, sequences that should take five minutes take seven or eight. Director BJ McDonnell seems to have had an ambition to make “Studio 666” work as a horror film, so he milks every sequence with diminishing returns.

McDonnell’s ambition was understandable. A movie like “Studio 666” is in danger of becoming one joke, played over and over, so any director might want to give the audience some other reason to keep watching. But there’s nothing in the script to support even a hint of seriousness.

The truth is, “Studio 666” reallyisjust one joke, and so McDonnell had only one play that he could make here — to take that joke, to hit it hard and keep hitting it, and then get out fast, while the audience is still laughing. He doesn’t quite do that. At 106 minutes, “Studio 666” overstays its welcome.

Still, if you see it, it’s not the extra 20 minutes you’ll remember. It’s the sight of Grohl throughout, who looks as though making a movie is the most fun thing he’s ever done in his life.

L“Studio 666”:Horror comedy. Starring Dave Grohl, Taylor Hawkins and Will Forte. Directed by BJ McDonnell. (R. 106 minutes.) In theaters Friday, Feb. 25.