I did this too...
You may find that the material has already been spoken for, or that the author/agent/publisher has unrealistic expectations, the moment the magic words "movie rights" are mentioned.
I've had two good and bad experience. The latter was when I asked for the rights to a Sci-Fi novella, that had a great premise for a film - the author wanted a fortune, and I let it go, as I knew it would be very uncertain whether the film would ever get made, it was a gamble (isn't it always?), and I didn't want to put that much of my own money into it before there was a script.
The other two instances were great. The publisher of one author was also acting as his manager, and he got in touch with the author, set up a meeting, and I got the rights, for scale.
The other instance was even better - the author loved the fact that I wanted to make a TV-series out of his book, and let me have the rights for free.
There's a mountain of material out there that's not been spoken for - and maybe you're the one who sees the unique angle the material possesses, and that can make it happen.
Don't promise the moon. Explain where you're from, what you want to do and what an uphill struggle it is to get anything green lit in our business. But be enthusiastic about why you want that particular property. Don't go into specifics about how you are going to do it or write it - as the author is bound to have very set opinions of his own, and may react negatively upon hearing your suggestions.
Discuss whether the author will want to approve the final script. (That may become and issue if you get the material on the cheap.) If you follow a "standard" contract, then the author relinquishes control over the movie content upon the property being transferred to you.
You'll also have to discuss how long you want the rights, and what, specifically, happens if a producer picks up the project.