Once I've done my research and established my characters, I sit down at the keyboard and write out my entire plot in about an hour, start to finish. The key to success in writing is focus. You have to have an end in mind so you can direct your work to that end. This is particularly true in category and to a lesser extent in the historical market. It would be foolish not to research the market in advance to know there's a place for the story you want to write.
Different houses have different expectations.
Do your homework. No, not directly. Fiction is just that, fiction. But every writer soaks up the world around them and draws from that whole to create. If anyone reads my work thinking to find themselves in it, they won't, but they may find a character who has the same habit of perforating Styrofoam coffee cups when nervous.