“I don’t know what I think of my work—I let the people tell me,” said Mexican-born director Patricia Riggen of her first feature, “Under the Same Moon,” about a young boy trying to cross the border to reach his mother, an undocumented immigrant working in Los Angeles, after his grandmother’s death. “I feel sometimes it’s like when you’re growing tomatoes, and you don’t really know how they’re going to come out. I really never know what I have until people see it. Then I get an idea of what it is.”

During an interview in Dallas, Riggen recalled her previous film, the documentary “Family Portrait” about poverty in Harlem, which she set aside for half a decade before finishing it, only to have it win the Grand Jury Prize at the Sundance Film Festival. “I’m thinking, oh my God, I had no idea! It was sitting in my closet for five years! It gave me a lot of exposure, and I didn’t know it was powerful until I saw an entire audience in Manhattan crying.”

Riggen was sent the script of “Under the Same Moon” by its writer, Ligiah Villalobos, who’d seen her film. “I had my doubts,” she admitted, “because it was an immigration story, and I thought, nobody’s going to want to see a movie about immigrants. But then I thought, that relationship [between a mother and son] is universal, and the separation of families is universal—it happens, sadly, all through history in every country in the world. Then it became this very, very hot topic.”

The picture was shot on a modest budget, and almost entirely in Mexico, “except,” she added, “for exteriors in Los Angeles, because exterior Los Angeles cannot be duplicated—we know it too well. The rest is Mexico. Even all the interior L.A. is in Mexico.” That, she laughed, required great care in continuity: when a character walked out of an apartment, shot in Mexico, into an exterior shot in California, every detail had to be exactly the same. Even a sequence at the US-Mexico border, complete with customs booths, was shot in Mexico. “That’s a warehouse entrance!” she said, laughing. “It’s better than a border crossing.”

Casting the picture, Riggen said, was challenging, and she took some risks in it. Carlitos, the young boy, is played by Adrian Alonso, known from “The Legend of Zorro.” Riggen recalled, “I read an interview with him in a political magazine from Mexico…and I thought, this kid is giving an interview like he was an adult. And I thought, that’s Carlitos—that’s exactly the kind of personality he’d have. He played a big role in ‘The Legend of Zorro,’ so he was known from that, and he had made several other movies. But when I auditioned him, I wasn’t sure that he was the right kid. Eventually I did some improvisations, and that’s when I realized how good he is. He’s so quick, very funny and very smart. That’s one of the reasons why I think the energy between him and Enrique [the character played by Eugenio Derbez] really works. He’s not a little, weak child—he’s strong.”

The character of Enrique is a surly, independent fellow to whom Carlitos attaches himself on his journey to L.A., and of course the two bond along the way, and Riggen’s casting of Derbez was even more daring. “He happens to be the most famous comedian in Mexico,” she said. “He’s huge. And the interesting thing is, he’s never been in film and never done a dramatic role [before]. He’s been trying to get into films, and nobody had given him a chance. I did. He was very risky, I have to say. And I think he did a wonderful job. The challenge in Mexico was how to make them see a character—not Eugenio, but Enrique.”

Kate del Castillo, who plays Carlitos’ mother Rosario, also represented a risk. “She was a challenge of perception,” Riggen explained, “because she comes from soap opera in Mexico, from telenovelas. She was the queen of the telenovelas. I think people are going to discover a Kate they haven’t seen before. She does a beautiful job. She doesn’t get the journey—the kid gets the journey, he gets the adventures, all the encounters. She’s sitting in one place thinking about her problems, her dilemmas. That’s very hard to pull off, and to be lovable.”

It wouldn’t be fair to reveal how “Under the same Moon” ends, but Riggen’s remarks about her goals offer a hint. “What I do in my work…[is] to create emotion. Not from sadness, from making a depressing film where the kid has cancer and dies—I don’t like that. I like to create emotion from joy.”