Bill Murray has a honking fat role in “St. Vincent,” his biggest part in an out-and-out comedy since “The Life Aquatic with Steve Zissou.” That’s pretty much the sole draw for the movie, and given Murray’s unique screen presence, it’s something.

He really looks juiced in this one, doing loose-limbed dances — his great ungainly body remains a vehicle for endless comic possibilities — and bellowing out insults to friends and enemies alike. He even remembers to adopt a New Yawk accent at times.

If it were a better movie, this would be a signature role, because it’s all about the Murray persona: a deeply sarcastic man struggling to find his way to sincerity. That struggle is why Murray looks so melancholy in so much of his work.

But it’s not a good movie. Murray’s slovenly misanthrope is Vincent, who reluctantly agrees to babysit the son (Jaeden Lieberher) of his new next-door neighbor (Melissa McCarthy, toning it down here). This will take time away from drinking, gambling at the racetrack, or visiting his Russian prostitute (Naomi Watts, getting the most out of her accent), but he’s light on cash at the moment.

Vincent will find a way to include the kid in all of these activities, and the two of them will learn lessons about each other. We are also cued to the reasons Vincent is curmudgeonly, none of which will come as much of a surprise.

Writer-director Theodore Melfi tries hard to convince us that Vincent is capable of great nastiness, but even these efforts seem rigged to ultimately show the soft, gooey center of both character and movie. As much pleasure as I took from watching Murray stretch out, I didn’t believe a minute of it.

“St. Vincent” has one great sequence, and it comes during the end credits. In ordinary circumstances this might be a spoiler alert, but the film’s distributor has already released the scene as a quasi-trailer, so whatever. The sequence consists of Murray sitting in his backyard, futzing around with a hose and singing along to a great Bob Dylan song.

That’s the extent of it — it serves no narrative function and exists only as pure character observation. And thus it is well suited to Bill Murray, an actor so often superior to the movies around him.

“St. Vincent” (2 stars)

A big role for Bill Murray is the main draw in this familiar comedy; he’s a curmudgeon who babysits for his new neighbor (Melissa McCarthy). It’s just not a very original movie, even if Murray is fun to watch. With Naomi Watts.

Rating: PG-13, for language, subject matter

Showing: Meridian