He plays a radio journalist who finds himself caring for his young nephew when his sister needs some help.
Phoenix plays uncle to a boy who spouts out lots of question. Some might provide a few answers for the older guy when it comes to family, love and bonding with a child.
Woody Norman, who plays the boy, doesn't like revealing too much about the plot. When people ask him what the movie is about, he's a little vague.
"I just say it's about a uncle and nephew going on a journey throughout three different parts of America," Norman said.
On casting the young actor, writer-director Mike Mills said: "Woody has such a, like, deep presence and knows how to not overperform for the camera and just follow his instincts."
Mills is equally happy having Gaby Hoffman in his movie, who plays the boy's mother.
"Mike found me. Lucky me," said Hoffman. "We had a very lovely long dinner where we started talking about all the things that life is about and this movie is about. And I just wanted to keep talking. So here I am!"
"These are things I'm interested in: love, trust, family, compassion, the act of loving in all of its difficulty and beauty," she added.
Mills chose to make this movie in black and white because that's how he envisioned it from the start.
"And black and white kind of pulls us out of reality and puts us in, like, more of a story space or, like, an art space," he said.
"C'mon C'mon" is in theatres now.