The stars are aghast – Hollywood's juiciest new roles are going to unknown actresses

For Phyllida Lloyd, the decision must have been straightforward. Not only has she worked with Meryl Streep before, but she knows that – in casting the Oscar-winner as Margaret Thatcher in the forthcoming biopic Iron Lady – she is almost guaranteed a hit. Streep, after all, is both extremely famous and extremely bankable. Last year Forbes named her the seventh best value-for-money star in Hollywood. The third woman on the list, she was trumped only by Anne Hathaway and Jennifer Aniston. She is, at least on the face of things, a pretty sound bit of casting.

But what of Adrianne Palicki? While Streep transforms herself into Maggie, Palicki is preparing to become Wonder Woman. A relative unknown, the Ohioan actress was last week named as the star of a new adaptation of the 1970s classic. It's a casting decision which had young Hollywood on tenterhooks. Scripted by David E Kelley of Ally McBeal acclaim, Wonder Woman is set to become television's next big hit. Christina Hendricks, the star of Mad Men, made her interest in the part known when she told an interviewer that she had "been wanting to wear that outfit (her) entire life". Beyoncé has said that she hopes to be the first Wonder Woman on the big screen.

Palicki isn't the only unknown entity to secure herself a plum job of late. When it was announced that Rooney Mara would take the role of Lisbeth Salander in David Fincher's adaptation of the Stieg Larsson's book series, The Girl With the Dragon Tattoo, Hollywood-watchers were confounded. It was one of the most sought-after roles of the year. Emma Watson, Carey Mulligan, Scarlett Johansson and Natalie Portman had all been said to be in the running for it. And yet, instead of opting for a tried-and-tested silver screen name, Fincher cast a girl whose career has thus far been limited to a leading role in the poorly received remake of A Nightmare on Elm Street and a bit-part in The Social Network.

Of course, there is one very obvious advantage to casting a newcomer. They will cost considerably less than Hollywood royalty. It's not known what Mara will be paid for her efforts, though it's safe to say it won't be the £10million that Emma Watson earned for each of the final two Harry Potter films. And she won't rack up the expenses of other stars, according to Nina Gold, who cast Colin Firth in The Kings Speech: "It's not just about what they are paid – there is all sorts of stuff you don't see on screen. For some stars, studios end up paying for everything from their trainer to their dog-walker."

But cost-cutting doesn't appear to be Fincher's motive in casting Mara – even if Daniel Craig is said to be earning £12m for his role alongside her. Picking a new face allows filmmakers to work with an actor about whom the public has no preconception. Instead of having to get past the barrier of their celebrity, with all the gossip-magazine baggage and previous role associations that it may bring, directors can simply present their star to the audience in the mode of the character. "If you have a very famous person in the lead role, a lot of people find that it is all they will see," notes Dr Warren Buckland, Reader in Film Studies at Oxford Brooks University. "It creates a lack of sincerity." To this end, witness Mara's recent photo shoot with W magazine, in which she was styled to resemble her fictional alter-ego – complete with multiple piercings and trademark tattoos.

Added to this is the inevitable kudos awarded to a film which discovers a new star. "Often the most exciting roles are the ones that launch a new name," observes Nina Gold. "When it all comes together, and the actor is perfect for the part, there's really nothing like it." There is little doubt, for instance, that the novelty of two brand new talents, Freida Pinto and Dev Patel, contributed to the buzz surrounding Slumdog Millionaire. And in casting the relatively obscure Kristen Stewart and Robert Pattinson as teenage star-crossed lovers, the makers of the Twilight Saga ensured that the two young actors will be forever associated with the series. Indeed, says Buckland, for many a filmmaker this temptation is too great to resist. "Every director dreams of discovering the next big star."

Perhaps, then, casting a newcomer is not quite the blind leap it may seem. After all, while Meryl Streep et al might enhance the chances of a film being a hit, they can't guarantee it. Look what happened with Lions for Lambs. And, of course, as Buckland points out, there is one major disadvantage of working with an A-lister: "They're the boss. The director no longer has control – it all becomes about protecting the celebrity's image."

In an age when studios and investors are looking to maximise box-office success and minimise financial loss, launching a brand new, DIY star might just be the best bet.