The results from a survey conducted by the audience research firm Crowd Science (published on Sept. 21, 2011) suggest that - in general - online ads for your film make the most sense if your target audience is older: "Adults aged 55 and older click on online ads more when compared to other age groups."
What sort of online ads worked for this aging audience? Apparently consumers often clicked on ads that showed them products already of interest. So, while you can use paid ads to introduce your film to potential consumers - to make a sale you may need an established "brand presence" in your target audience (and establishing a brand presence can be very time-consuming and costly).
The Crowd Science study suggests that the reasons for not clicking on ads varied by age and gender, but the majority of users who declined to click did so based on perceived lack of ad trust or relevancy.