Kids can be safely relied on to say the darnedest things, and in Mike Mills’ latest feature, indeed they do. But they also say everything else – the funniest things, the saddest things, the strangest things, the most wondrous things.

Precocious little Jesse (cherubic Woody Norman) often manages all of this at the same time, his youthful lack of filter combining with his born sensitivity in such remarks as when he informs uncle Johnny (Joaquin Phoenix) that the man’s not very good at expressing his emotions.

The deceptive maturity of children, when played against the limits of how much an adult can fairly expect on that front, makes for gentle drama in Mills’ study of a hurting family’s unconventional healing. Johnny has built his career around the concept that our offspring know more than we realise, though he has none of his own; Phoenix portrays the host of a radio show who travels the country interviewing young people about their lives.

On paper, he’s the ideal temporary guardian for Jesse once Johnny’s sister Viv (Gaby Hoffmann) needs some time for help bipolar husband Paul (Scoot McNairy) through an episode of instability. But all Johnny’s curiosity and empathy can’t prepare him for the practical challenges of time management in childrearing, or the possibility that Jesse may have inherited some of his father’s behavioural patterns.

This falls into the tradition of movies about closed-off guys learning to feel from the innocents left in their charge, but Mills skirts triteness by softening Phoenix’s usual intensity. He’s not starting from a place of emotional rigidity, as made clear in his treatment of the subjects in his polite, studied recording sessions. Instead, he’s merely a bit sad and lonely, two inner holes filled with the energising purpose of surrogate parenthood.

The black-and-white photography is Mills’ big formal choice, its effect not disagreeable despite hard-to-discern motivations. In practice, cinematographer Robbie Ryan’s monochrome has a way of making disparate pockets of America look unified, despite the location shooting bringing out their individual beauty.

Jesse has an odd pre-bedtime routine of pretending that he’s an orphan, talking about the contemptible conditions at his orphanage, and asking if he can’t stay the night in his own bed. It weirds Johnny out the first time he sees it, and the audience is right there with him, but we both come to understand the rationale.

In this film full of people hesitant to open themselves up, acceptance becomes a naked plea that demands to be made every day. The tone never defines the stakes in such grave terms, but that’s the key to the potency of Mills’ cinema: life’s pivotal turns come in idle moments, from inconspicuous sources. All it takes is the willingness to listen.