When it was revealed that video of Charlize Theron’s reaction to 2013 Academy Awards host Seth MacFarlane’s awards-night ditty “We Saw Your Boobs” was not spontaneous exasperation but a pre-recorded part of the joke, we learned two things about the Oscar-winning actress: She’s not only a really good sport, but, boy, can she keep a straight face.

Both of these skills are in evidence in “A Million Ways to Die in the West,” a broad, wildly hit-or-miss satire of Westerns co-starring Theron and MacFarlane (who also directed, produced and co-wrote the film). Crude, rude and — in the case of actor Liam Neeson’s derriere — briefly nude, the movie not only taxes Theron’s willingness to crack wise about her lady parts, but demonstrates that she can do so without cracking up on camera.

The same cannot be said of MacFarlane.

Look, no one ever accused the guy of being Sir Laurence Olivier. Best known as the creative jack-of-all-trades behind the cartoon series “American Dad,” “Family Guy” and “The Cleveland Show,” MacFarlane, when put in front of a camera, seems caught in a perpetual smirk, as though he’s inordinately pleased with himself.

Perhaps he has some right to be. The new movie — in which MacFarlane plays a cowardly, lovelorn sheep farmer, with Theron as his romantic mentor and Neeson his gunslinging nemesis — is passably funny, assuming you have the stomach for jokes about diarrhea, sex, death and all manner of body parts (both human and sheep). Judging by the rapid-fire, raunchy humor in the red-band trailer, this is an effective combination. When stretched out to almost two hours by all the absurdist plotting, pop-culture asides and sometimes painfully awkward silences that MacFarlane is known for, it’s not quite so zingy.

In fairness, MacFarlane knows his own weakness. He’s not the hero of the movie, as his character, Albert, reminds us. Rather, he’s “the guy in the crowd making fun of the hero’s shirt.” This observational detachment — being in the movie, but not of it — can work, but only to the degree that you let it. As the Village Voice favorably — and aptly — noted, MacFarlane is like the animated Roger Rabbit here, a cartoon stuck in a live action world.

Some viewers, however, may wish for a bit more commitment.

The disconnect between Albert and the world he inhabits is at times off-putting, especially when you compare MacFarlane’s performance with those of his co-stars. Theron is not only quite funny, but also fully invested in the role of Anna, a woman who pretends to be Albert’s girlfriend so that he can make his ex-squeeze, Louise (Amanda Seyfried), jealous. Neil Patrick Harris also fully inhabits the role of Louise’s extravagantly mustachioed and egotistical lover, Foy. Though shallowly written, both parts are grounded in reality.

With a supporting cast as strong as this — supplemented by several surprise cameos from stars I won’t mention — does anyone care that MacFarlane can’t really act? It never hurt Jerry Seinfeld, whose paper-thin persona on his self-titled sitcom was made plausible, if not fully three-dimensional, by the top-notch ensemble that the star surrounded himself with.

No, the problem isn’t MacFarlane’s inch-deep performance. In a way, we need him to splash through the proceedings like a kid in a puddle, making fun of such tropes of the Western genre as the barroom brawl, the bordello girls with hearts of gold, the perfectly choreographed barn-dance blowout and the climactic gunfight. Cutting away from the action to comment on it is the joke. And it has become MacFarlane’s signature style.

The film’s failure isn’t one of restraint, either. As he demonstrated with his “Boobs” song, MacFarlane’s gift, such as it is, lies in pushing humor to the threshold of decorum and then charging across that line, consequences be damned. That’s what we pay him for.

The real problem with “A Million Ways to Die in the West” is one of editing. There are a million jokes in it, but only 500,000 of them are funny.

★ ★ ½ R. At area theaters. Contains crude sexual and excretory humor, obscenity, violence, drug use and brief nudity. 116 minutes.