Rarely does a debut novelist arrive with such a pedigree as the writer, director, animator, producer and principal voiceover artist of Family Guy, American Dad! and several other cartoon series, as well as of Ted, the highest-grossing adult comic film ever made. Seth MacFarlane is also the highest-paid television writer in history, having bagged a $100m contract with Fox.

He can now add novelist to that crammed CV, but he does so with a very strange book. It is a comedy western, for a start, and there are not many of those around, thank God. It is also, oddly, a spin-off, being the novelisation of a forthcoming film, co‑written and starring MacFarlane in his first acting role (another string to his ludicrously thick bow).

In the film, MacFarlane plays the hero, Albert Stark, a lily-livered coward who is completely out of place in the wild west of 1882. Albert is permanently terrified, and with good reason. "We live in a terrible place and time," he laments. "Everything that's not you wants to kill you. Outlaws. Angry drunks. Scorned hookers. Hungry animals. Diseases. Major injuries. Minor injuries. Indians. The weather." Basically, Stark is a modern guy teleported to 19th-century Arizona, coming up against the grim realities of the olden days and being all, like, "Eew".

Albert's cowardice has helped to keep him alive, but it costs him the love of his girlfriend, who runs off into the arms of the owner of the local moustachery. Help comes from the unlikely source of a stranger in Albert's one-horse town – Anna, a glamorous, mysterious moll. Albert and Anna share a 21st-century vocabulary and bafflement at the awfulness of their times, but when they fall in love it is for no apparent reason other than the narrative's demand.

Alas, Anna is already betrothed, and, even more alas, to Clinch Leatherwood, the deadliest outlaw in the west. In a very strange scene towards the end, Albert prepares for the inevitable high-noon duel with Clinch by consulting the local Apaches, who give him drugs which send him on a 10-page psychedelic trip. It is a weird digression to an oddly jerky storyline, but by far the strangest feature of this novel is that it is almost entirely unfunny. Just why that should be isn't immediately obvious. The basic anachronistic premise is terrific. MacFarlane obviously has an excellent ear. And I can quite see how some of the scenes might be genuinely amusing in the film – those set in the local brothel, for instance, where Albert's best friend is a shy, sweet suitor conducting a shy, sweet, celibate romance with a shy, sweet and very busy prostitute. But, like the rest of this book, I had to plough through those brothel scenes with the glum doggedness I usually reserve for tax returns.

So what has gone wrong? Part of the problem is that MacFarlane doesn't seem to have added too much to the film script, so this is a novella-length novel and a noticeably flimsy one, lacking detail and context. And what writing there is just isn't sharp enough to make the jokes work on the page. The laziness of the prose results in, for example, the following descriptions of various women – "beautiful", "very pretty", "quite beautiful", "strikingly beautiful", "a rare beauty", and "the most beautiful woman in the world".

The failure of the comedy is such that the best bits of this comic western come from the western element – a stagecoach robbery, a jailbreak, the final shootout. Not that they're anything to rival early Elmore Leonard, but there is a vividness about the action sequences that has simply gone missing in the supposedly fun stuff. Truly bizarre.