I love writing dialogue and I have actually learned the difference between these two things. It helps that my husband has a BA in Cinema. A screenplay is the road map or basic layout and ideas for a movie. If you were to adapt a book to a movie format you would have to shorten the story line to three main parts, 1 beg 2 climax, 3 finale. A script consists of actual dialogue the actors will memorize and perform. A screenplay will have dialogue but it is not a final product. Instead, a screenplay describes scenes background information and motivation. The best guide would be a How-To book. Remember the only people that fail are the ones that never try. Good Luck!