Actress Anne Hathaway has grown up quite a bit – both onscreen and off – since making her feature film debut in Garry Marshall's family friendly film, The Princess Diaries , way back in 2001. At the age of 18, Hathaway was cast as an unpopular, klutzy high-school student who, during an unexpected visit from her prim-and-proper grandmother (played by the classy Julie Andrews), finds out that she is, gulp, a real-life princess!

After following up her role as Mia Thermopolis in Royal Engagement , the 2004 Diaries sequel and starring as the title character in Miramax's sweet live-action fairytale, Ella Enchanted , Hathaway, at 23, is currently making it a point to expand her resume. Continuing to push tyke-targeted fare aside, Hathaway – who immersed herself in Ang Lee's drama-packed Brokeback Mountain late last year – is presently starring opposite veteran actress Meryl Streep in The Devil Wears Prada .

The film, directed by David Frankel (whose resume includes helming episodes of both “Entourage” and “Sex and the City”), is adapted from Lauren Weisberger's book of the same name, which spent a whopping six weeks at the top of The New York Times bestseller list a couple of years ago. Weisberger apparently based the book on her own experiences working at Vogue magazine, where she served as an assistant to editor Anna Wintour – a woman who is rumored to be one of the toughest, most demanding people in the magazine industry.

The brash, heavy fisted editrix in The Devil Wears Prada is named Miranda Priestly – who runs the show at the fictional Runway magazine – and is played with powerful panache by a sleekly coiffed Streep. Dressed head-to-toe in trendy designer clothing (namely Prada, hence the film's title), Priestly, who is extremely demanding, finds that she cannot hold on to a good assistant. That's when dowdy, aspiring journalist Andy Sachs (Hathaway) enters the picture.

Fresh out of Northwestern University, Andy has moved to New York City in hopes of finding a job in publishing. While Priestly instantly dismisses Andy – flat-out telling her that she has no fashion sense – the young Miss Sachs refuses to take no for an answer. Her intelligence and diligence impress Priestly, who agrees to test-drive Andy as her assistant.

“Andy begins as a strong, grounded character, and as the story progresses, she becomes more and more preoccupied with success,” says Hathaway, whose character eventually transforms herself from a sweet, persistent journalist into a somewhat conceited, designer-loving label queen. “All she can see is her job and doing it well. When you've imagined that you're going to save the world, and you find that the world isn't always willing to be saved, you need to rethink who you are.”

Adds the actress, “If you're over age 21, you've probably had some of Andy's experiences.”

One of Hathaway's experiences while prepping for Devil involved Frankel telling his young actress that she needed to put on about 10 pounds for the role. According to the director, this was important to illustrate the distorted aesthetics of the New York fashion scene, where someone like Andy – who's probably a size 6 instead of a size 0 – really stands out.

“She is a beautiful girl who is not model-thin, which is an important distinction for this character,” says Frankel.

So, how did Hathaway put on the excess weight?

“I ate pizza, beer, ice cream – the college diet,” she recently told US Weekly . “I condensed the freshman year into just four weeks.”

Hathaway, who was educated at both Vassar and New York University, can apparently do more than just reverse diet. According to producer Wendy Finerman, the actress – who just wrapped the period piece Becoming Jane , in which she plays a young Jane Austen – also has an enlightened knack for performing comedy.

“Anne is funny and will do things in a way that not a lot of actors could do, and every time she does so, it is fresh and new,” says Finerman. “She finds the funny and never holds back.”

Oscar-winner Streep, who, in addition to The Devil Wears Prada , is starring in this month's Robert Altman-helmed A Prairie Home Companion , also has a few kind words about her young co-star's abilities.

“There's no artifice about her,” says Streep in a recent article published in Entertainment Weekly . “Anne is just sort of buoyant, and it's a lovely quality for this character who could be annoying. She's just darling.”

And while Hathaway has a ton of positive praise to bestow upon Streep, her character, Andy, doesn't feel the same way about Priestly – at least at first.

“Miranda is cold, has no feelings and only focuses on one thing: her job,” says Hathaway, whose character's opinions about her seemingly cold-hearted boss eventually begin to change. “Andy learns from Miranda the relentless pursuit of journalism excellence. She teaches her what it takes to be the best.”

Hathaway also notes that, even though Devil is set in the high-stakes, somewhat surreal world of fashion, her character's journey as Miranda's assistant is universal in its theme of a young person finding her own way in a very confusing world.

“What happens to Andy is very character-forming,” says Hathaway. “As she changes, she realizes it's important to commit not necessarily to success, but to succeed on your own terms.”

With The Devil Wears Prada almost out in theaters and Becoming Jane already in the bag, it seems as though Hathaway is doing just that – she's succeeding on her own terms. She is scaling the brick wall that was built by her roles in kid-friendly fare ( Princess Diaries , Ella Enchanted and the animated comedy Hoodwinked , for which she voiced the character of Red), and taking on riskier, more adult-themed projects like Brokeback Mountain , Devil and a direct-to-video drama called Havoc , released last year, in which she stars alongside Bijou Phillips and Joseph Gordon-Levitt.

If Hathaway stays on track, she'll definitely succeed in expanding her resume – and continue surprising audiences along the way.

The Devil Wears Prada releases in theaters June 30.