By Phil Boucher and Julie Jordan
Updated June 25, 2012 12:00 PM

Just call it Grandma to the rescue. Soon after adopting son Jackson, 6 months, Charlize Theron faced a daunting challenge: promoting two blockbuster films around the world. Fortunately her mom, Gerda, 59, stepped in immediately to help with late-night bottle feedings and mid-flight diaper changes. “She has taken to the role of grandmother like a fish to water,” says the 36-year-old actress, who adopted Jackson domestically after a two-year process. “I have moments where I look at her and go, ‘Were you this nice to me?’ It heightens your relationship, which is really bizarre because I didn’t think I could get any closer to my mom,” she tells PEOPLE. “I would not be able to do this without her.”

Especially because the Oscar winner is in the midst of an ultrabusy summer: Her two new films, Snow White and the Huntsman and the sci-fi thriller Prometheus, each opened to huge numbers at the box office. And she continues to win raves for her deliciously villainous turns in both. To play the psychotic evil queen in Snow White, Theron worked with a dialogue coach two hours daily, honing a British accent not just when she spoke but when she screamed as well. “It was funny, because I realized that nobody would ever come to my hotel room while we were working,” she says. “When I left, everybody kind of looked at me like I was absolutely crazy.” For her role as a steely executive in Prometheus, she displayed another kind of commitment: slipping her famous body into a skintight space suit. “It would tear and [the costumers] would actually come and super-glue it onto your body,” she recalls. “I was like, ‘Really? We’re doing that?’ ” Despite the arduous shoot, Theron maintained a relaxed vibe off-duty. “She’s got a fairly foul mouth on her,” says costar Guy Pearce. “So she’s fun to be around.”

As Theron’s career continues to explode-she recently shaved her head for her role in Mad Max: Fury Road-the actress will no doubt be counting on Grandma even more. “Right now, like any working mom, I’ve been just trying to figure out how to juggle everything,” says the star, who split from actor Stuart Townsend in 2010. “But I know what my priorities are. This is everything that I’ve dreamed of and about for a really long time.”