Take a look at the jQuery plugin BlockUI. It will allow you to block out a page and show whatever content you want to. See the site for all of the options that are available, inparticular take a look at the Page Blocking tab. What you'd do is have a DIV that contains your moive and your form.
The problem you're going to have is to figure out how to identify folks that have filled out the form. One way would be to use SecurityAssist to create a log in page, and then those that have logged in don't see the movie, or you could use Cookies Toolkit to set a cookie when the form is submitted and then check for the cookie before showing the movie.