Jeff Nichols’ last film, “Taking Shelter,” was about a man trying to protect his wife and daughter. Before that, he made “Shotgun Stories,” in which two sets of half-brothers feud after their father’s death. His third film, “Mud,” takes a 14-year-old boy’s point of view, but it is once again about the pressures of manhood and the importance of father figures. Nichols is examining the subject of modern masculinity more richly and thoughtfully than any other filmmaker.

He’s doing a pretty entertaining job of it, too: “Mud” is warm, humorous, and smart, a Huckleberry Finnish snapshot of carefree boyhood on the brink of adolescence. Our unassuming young hero is Ellis (Tye Sheridan), an Arkansas boy who lives in a houseboat with his parents (Ray McKinnon and Sarah Paulson) on the banks of the Mississippi River, where Dad makes a hard living as a fisherman. Ellis and his best friend, Neckbone (Jacob Lofland), in the course of their boyish summertime explorations, find a vagabond (Matthew McConaughey) living on a small island in the river. His name, he tells them, is Mud. He’s not a permanent resident of the island, just lyin’ low for a spell, waitin’ to meet up with his girlfriend. He has a gun and a lucky shirt. He tells tales that might be a little tall.

Every boy needs a character like this in his life. It can be an uncle, a neighbor, even a hobo who lives on an island in the river and might be a fugitive from the law. It just needs to be someone who will look out for the kid and teach him about life by saying and doing crazy things, things that aren’t said and done at home. Even boys like Ellis, who have loving, responsible fathers, need an outside influence like Mud for guidance and advice. You know how they say it takes a village to raise a child? Well, that village should have some fun-loving, semi-employed, young-at-heart but harmless ne’er-do-wells. I strongly believe this.

Anyway. Ellis, aided by Neckbone, runs errands for Mud, keeping an eye out for Mud’s girlfriend, the reportedly beautiful Juniper (Reese Witherspoon), when they’re in town. At home, Ellis’ parents are splitting up, for ordinary reasons. “You can’t trust love,” his dad says to him. But Ellis isn’t so sure. He sees Mud living like a person who believes in nothing but love, risking everything to be reunited with Juniper. It’s all so … swashbuckling. And it makes an impression on a river boy. When Ellis sees the object of his own affection, a girl named May Pearl (Bonnie Sturdivant), being pestered by schoolmates, he rushes in manfully and throws a punch to defend her honor.

Life is a little more complicated than that, as Ellis comes to find out, as adolescents tend to find out in coming-of-age stories. “Mud” is marvelously sensitive in its depiction of growing up, and in particular the way that male role models shape a kid’s perception of the world. Ellis has his father and Mud, one realistic and responsible, the other more adventurous; Neckbone’s parents are gone so he lives in a trailer with his white-trash uncle, Galen (Michael Shannon), a no-account womanizer; Mud describes old Tom Blankenship (Sam Shepard), a grizzled loner who lives across the river from Ellis’ family, as “closest thing to a daddy I ever had,” and Tom proves a valuable mentor later on. This whole chain of events, we come to find out, was set in motion by a father and son moved by filial devotion.

Nichols slightly loses his grip on the plot near the end, letting it run loose with implausible action and too-tidy resolutions, but not enough to do any serious damage. McConaughey, his career McConaissance in full bloom, has been deservedly praised for playing Mud as a larger-than-life yet eminently believable figure, and Shepard, Shannon, Paulson, McKinnon, and Witherspoon all deliver in their supporting roles among the grown-ups.

But the film belongs to the young people. The boys in “Mud” are interesting because they’re ordinary. Some of their circumstances are strange, but the boys themselves always seem like average American kids, which makes them relatable. They’re the boys we knew, or were. Tye Sheridan, who plays Ellis, had only been in school plays before he got a part in Terrence Malick’s “Tree of Life” (his one pre-“Mud” credit), and Jacob Lofland was a local kid plucked from obscurity at an Arkansas casting call. But a lack of experience doesn’t mean a lack of skill, and it’s harder than it looks to convincingly play a character who is average. Average people do things like cry, worry, laugh and fight, all of which look fake if the actor doesn’t know what he’s doing. The most emotionally fraught scenes in “Mud” show that Sheridan is the real deal, able to go much deeper than just playing make-believe pretend time. (Lofland is good too, but doesn’t have nearly as much to do as Neckbone, even though a character named Neckbone obviously should have prominence.) You heard it here (and in a million other reviews) first: Tye Sheridan has the raw talent to become a great actor. The kid is going places, especially if he keeps working with perceptive filmmakers like Jeff Nichols.

B+ (2 hrs., 10 min.; PG-13, some profanity and adult themes.)