The 26-year-old, whose parents live in Gledhow, plays Lisa, a fun-loving lass who comes, strangely enough, from Leeds.She is with two girlfriends on a carefree holiday in Spain which spirals out of control when they meet four charming young men.
The film, which was premiered in London's Leicester Square, was among the nation's top 10 most watched films in its first week.
One reviewer has called it "the most controversial British film since Trainspotting" and another said it was the "sexiest, most shocking thriller of the year."
Sian hopes it will be a springboard for her career.
Previously, she has done lots of theatre work and appeared in The Bill last January.
She will also be seen in an episode of the Yorkshire hospital soap The Royal, playing a heavily pregnant woman who goes into labour early.
Sian said of her big screen role: "Donkey Punch was well received at the Sundance Festival, and then at the Edinburgh Festival.
"Filming it was such a wonderful experience. I learned an incredible amount and the team became very close. We are all incredibly proud of the film.
"It was a low-budget film production was shot over six weeks out in Cape Town, South Africa. We expect it will be shown on television and will come out on DVD in November."
Sian, 26, studied at Roundhay School from 1993 to 99, staying on until she was 19 so she could do an A-level in drama.
"It was a new course," she said, "and so I did an extra year at school. We did a lot of plays at school and they fed our interest in drama.
"The teachers were really supportive and gave you confidence and if it hadn't been for them I might have done something else."
Sian left school to do a year of workshops at Stratford-upon-Avon and then went to drama school for three years.
Although she lives in London Sian frequently returns to Leeds.

Film extras needed for Leeds movie