CANNES, France — With a 2-year-old son and another child on the way, actor-director Diego Luna has been thinking a lot about fatherhood lately.

"Abel," the "Y Tu Mama Tambien" star's freshman fiction film, looks at the emotional fallout of a father's disappearance on his 9-year-old son. The bittersweet comedy — in which the young protagonist recovers his speech after retreating into muteness when his father walks out on the family — debuted at the Cannes Film Festival yesterday.

"It's a reflection on paternity. ... Becoming a dad really got me focused on the issue," the Mexican actor said in an interview.

"In Mexico, this idea that fathers go away is really deeply accepted because for so long so many men have had to leave to work in the United States," said Luna, who is married to Mexican actress Camila Sodi.

The couple is expecting their second child in several weeks.

"Those fathers miss an incredible journey: making decisions about what your kid eats, what kind of games he plays."

"Even things like changing diapers — my favorite thing," he joked.

Even on set, Luna ended up playing a kind of fatherly role, catering the production to the film's young stars, played by two non-actors — and real-life brothers — from the central Mexican city of Aguascalientes.

The boys, aged 6 and 10 during the shooting, were chosen from about 400 hopefuls who responded to a TV ad Luna placed, he said.

To help the boys get into their roles, Luna shot the film in chronological order, doling out the plot in bite-sized doses ahead of each day's shoot. On weekends during the 8-week-long production, he took the boys on field trips to the zoo and a children's museum, Luna said.

It paid off. Both boys deliver moving performances — particularly Cristopher Ruiz Esparza, who is funny and touching in the challenging title role. When he recovers his voice, Abel starts acting like a father to his siblings, bossing his younger brother and older sister around, and behaving like a husband to his mother.

It's a tribute to Ruiz Esparza's performance that he is utterly convincing, even when delivering lines that are out of place in the mouth of a such a small boy.

"My biggest fear was that one day he would tell me, 'Hey, today I'd really prefer to stay home,"' Luna said. "So I had to make sure that every day there was something on the set that could make him happy to be there, a game, something like that every day."