First, Carey Mulligan is a strong actress. (I think I don't need to say more to explain about it. :p)
Secondly, Carey Mulligan has good appeal to female audience, and I will try to say more about it in below....
Sex thrillers (and other kind of upscale sexual films) need to have more female appeal to do real business. So the casts of those sexual films had better to have good female appeal, especially actresses. For instance, I think one of the reasons why Atom Egoyan's "Where the Truth Lies" flopped so badly was that THINKFilm didn't sell Alison Lohman enough. That said, I think Alison Lohman clearly hated "Where the Truth Lies"; she only did very small amount of publicity for the film. ( Kevin Bacon and Colin Firth worked very hard to do publicity, but both of them didn't have enough female appeal to carry an erotic film.)
Furthermore, Sony Pictures Classics tried to sell "Tamara Drewe" as an upscale sexual comedy, and the film flopped; it may be partly because Gemma Arterton and other cast members don't have enough female appeal...... Yes, I know Gemma Arterton has a very sexy boby, so as Megan Fox and Rosie Huntington-Whiteley, but their image are oversexualized that they may even TURN OFF some female audience. Therefore, sexy actresses aren't necessary the good fits for upscale sexual films.
On the other hand, all three main stars of "Chloe" had very good female appeal; "Chloe" could gross a respectful $3 million (despite of many bad reviews) and went on becoming Atom Egoyan's biggest moneymaker ever.
BTW, Variety reported that many US theatrical audience of "Lust, Caution" were women; "Lust, Caution" eventually grossed a respectful $4.6 million and did very well in DVD rental.
In short, if I am a producer of a sex thriller, I will definitely try to cast Carey Mulligan instead of Olivia Wilde, Scarlett Johansson, Gemma Arterton, Megan Fox, Rosie Huntington-Whiteley and many other sexy actresses. Afterall, Carey Mulligan has good female appeal, while other actresses with oversexualized images may turn off some female audience.... and needless to say, Carey Mulligan is a strong actress (with enough bankability to at least greenlight $20 million indie film).
