To be quite honest, I've been very blessed when I've worked with Hollywood. The studios that have purchased my work to be adapted to film have really liked the work and wanted to stay as close as they could to what the book was.
A typical agent in New York gets 400 query letters a month. Of those, they might ask to read 3-4 manuscripts, and of those, they might ask to represent 1.
By reading a lot of novels in a variety of genres, and asking questions, it's possible to learn how things are done - the mechanics of writing, so to speak - and which genres and authors excel in various areas.