“Frank” is almost two movies at once. One is lively, silly and snaps along splendidly. The second shakes its finger at the audience for having too much fun, slowing into a clumsy lecture about mental illness being nothing to laugh at.

The fact that the movie attempts a serious message isn’t what hurts it; in many ways it would have been fine if the script didn’t inexplicably start to completely suck.

There is so much to like about “Frank.” That’s why it’s so unfortunate that it largely left a bad taste in my mouth.

We begin with Jon, played by Domhnall Gleeson, an Irish everyman struggling between his white-collar hell and his inability to write music. One day, his ordinary world is shattered by a call to adventure in the form of an idiosyncratic band fronted by Frank, who never removes a giant head with a cartoon face painted on it (Michael Fassbender).

Maggie Gyllenhaal plays a brooding, psychotic foil, who is basically summed up in one quote: “Don’t touch my f-ing theremin!”

The band’s delightfully unhinged manager is played by Scoot McNairy. Watch for this actor, he’s going to be huge one day.

The performances are all untouchable. In fact, it is the character-driven first half where the film completely shines. The scenes are quick, funny, certainly dark, but enjoyable as it zips along. It’s like spending time with the band, getting to know them and their delicious enigmas from the perspective of an outsider trying to be cool — a perspective anyone can identify with.

The turning point in the film occurs at the halfway mark with the death of a major character. While this is the film’s first jab at emotional drama, it is largely a wasted opportunity.

When the band finally and predictably hits the road with its “Captain Beefheart” weirdness to change the world, the film’s pacing grinds to a halt and character development utterly vanishes. Their characters are set in stone now, and the motion picture makes no more significant changes.

It’s hard to enjoy the film’s latter setting of “Austin,” being clearly Albuquerque, with easily recognizable appearances and performances by Lauren Poole, Alex Knight, Abraham Jallad and Timothy Kupjack.

While pacing and the missing human elements are the most glaring mistakes, the major failing of the second half’s tonal shift is inconsistency. The film suddenly tries to preach that mental illness is super serious and cannot be taken lightly, and then flops back and forth in an attempt at dark slapstick, which mostly falters.

“Frank” is a decent film, but the writer should have spent more time on the second half of the script.

Graham Gentz is a columnist for the Daily Lobo. He can be reached at culture@dailylobo.com.