Like Anne Hathaway in Les Miserables before her Sienna actually went under the scissors for her new film role, turning to hairdresser Luke Hersheson to chop off her locks.
"Sienna's new look is for a film but it's actually something we had been planning for years," he told Vogue.
"We wanted to do something that felt a little French in its effortlessness with a touch of boyishness - tough yet sexy," the celebrity hairdresser added.
"In the end we opted for a 'bad' (but good bad!) bob with an undercut at the back and sides which feels super-cool when her hair is up in a bun - a little on the rebellious side."
If you're brave enough to get rid of your below-the-shoulder 'do this year, then a long bob is the best way to ease yourself in.
Check out the gallery before for hair inspiration from the rich and famous...
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