An interactive screenplay is just like an ordinary movie screenplay, except that instead of a linear list of scenes, you (or the writer you hire) will have to write independent bits and pieces which can be played in a potentially large variety of sequences. Not only does this make interactive screenwriting very complicated (as it is both imperative and excruciating to ensure consistency), it also makes it long: a typical screenplay for a two-hour interactive movie contains about six hours of material. Writing an interactive movie typically requires 4 to 6 months of work, depending on the author's level of expertise with the medium.
As far as I know, there are still no good tools on the market to facilitate non-linear writing. Some packages support simple branching stories, but that's it. For anything more complicated than that, you are on your own.
Not very many people can write a convincing interactive screenplay, or explain how to do it. I recommend you look up Lee Sheldon's writings on Gamasutra or his CGDC tutorials if you are interested in this topic; he is one of very few masters of the genre.