On 17 February, the 33-year-old actress will take over from Emma Stone as Sally Bowles in Sam Mendes' production of the hit musical Cabaret - and the first pictures of Sienna playing the famous role have been released.
The Roundabout Theatre gave fans a sneak peek of the Sienna's style transformation on Twitter. The red lips, the curled blonde bob... yep, Sienna is one sexy Sally.
- Roundabout Theatre (@RTC_NYC) February 13, 2015
The American Sniper star will appear on stage alongside Alan Cummings in the final six weeks of Cabaret's run until 29 March. Is she nervous? As Sienna admitted in an interview with the Today show, singing isn't something she's famous for.
"I've never publicly sang or danced unless mildly intoxicated so this is going to be an interesting experiment," she said.
"They've heard me sing, and Sally Bowles – the original play is based on a book by Christopher Isherwood – and the real Sally Bowles, who was a real woman, was supposed to be rough around the edges," she explained.
"Not the Liza [Minnelli] version. She's supposed to be not great, and that's the truth. So I'm doing that version."
We're sure her predecessors Emma (above) and Michelle Williams had a few tips on coping with first night nerves.
Love Sienna? Look back through her fashion history below...
Don't we all?