A movie with an upbeat message, “Words on Bathroom Walls” examines mental illness from a high schooler’s perspective.

Adapted from Julia Walton’s YA novel, “Words” chronicles the trials and tribulations of  Adam (Charlie Plummer), a bright teenager who plans to be a chef.

Those hopes are derailed when he has a violent episode in chemistry class, is expelled from school and diagnosed as a paranoid schizophrenic. Those afflicted suffer hallucinations and hear voices they believe are real.

“Schizophrenia I think a lot of people have a misinformed idea. For me as an actor it was interesting to get more information,” Plummer, 21, said, “but also as human being.

“I didn’t know very much about schizophrenia before reading the script. My only knowledge” — he laughed at the memory – “was in middle school. I’d done a play a fellow student had written about a therapist and my character was diagnosed with schizophrenia. That research was using YouTube and Wikipedia.

“After that it was something I was interested in. Stories about mental health were always something I was curious about.”

He was cast because the producers saw his critically praised performance as kidnapped John Paul Getty III in Ridley Scott’s “All the Money in the World.”

Unlike middle school, this time Plummer really researched. “I wanted to feel I had an arsenal of knowledge. As best as I could when I was on set I didn’t have to ask, ‘Does this make sense in terms of experience?’ I kinda knew. When someone has their first episode of psychosis this is what happens. For me I wanted to have a security blanket I guess.”

Plummer bonded with his movie parents: Molly Parker as Adam’s fiercely protective mother and Walton Goggins his stepfather. “We went in a weekend before we started to film to be a family.”

Moviegoers see Adam with real people like the strict nun at his new school and simultaneously with the trio of imaginary characters who rarely leave unless he’s on his medication.

Filming with the two sets of actors surrounding him, Plummer said, “Just makes my job a little easier. That’s what the character is going through. I didn’t have to imagine.

“As a character, my mom is a real person — that’s the person I should be looking at when she’s speaking to me! But I also have these visualizations and as an actor I could pick those moments or in the scene itself maybe glance over at them.

“Honestly, it was like having a real playroom because there was a tone of playfulness. And real freedom.”