The "bits and pieces" hold a story together. This can be the hardest part of writing, at least for me. I usually know the beginning, the middle and the end, but the filling escapes me.
So, how do you know what to put in and what to leave out? Do you need to know while you are writing the story? I'll answer the second question first. No, if your story is flowing don't interrupt by trying to leave something out. It's better to have too much than too little. Remember, you can always remove it later.
Remember your story should have a series of ups and downs for your protagonist. As in real life nothing runs smoothly and let's be frank - Your reader doesn't want a smooth ride. It's the bumps that make the story interesting. Your reader wants to know (see) how your protagonist overcomes these trials.
How do you create these bumps (pitfalls)? For me it's easy, I just throw in some vampires. Voila! Built in bumps... Well, actually, this is the easy way out. I needed to create drama for my protagonist, so I gave him a villain to interact with throughout the story. The villain can be real or perceived... I think Snape is a good example of this. He is only a villain in Harry's mind and the readers via Harry Potter. He really isn't the bad guy he has been made out to be in the end. Maybe in the back of your mind Snape couldn't be bad, since Dumbledore trusts him, but you are unsure, until book 7 when all is revealed.
So, by throwing in Snape - J.K. Rowling created an extra layer to her stories. Though he really wasn't a bad guy, he, also, didn't like Harry that much either. Thus, creating friction for the protagonist.
Whatever road blocks you toss into your protagonist's way, he/she should be able to overcome them at some point. These smaller twist and turns prepare your protagonist for what is to come.
I hope this helps. For me reading other works has helped prepare me for writing. Though, it wasn't until this past November, when I wrote John Dunham's Diary, that I was able to accomplished this. There are still a few kinks to be worked out with the novel, but I know what I want to do. The important thing is that I finished it!
This accomplishment means more than all the previous starts that I have. It, also, allows me some insight on the problems that Twilight Dance has, so that I might be able to fix it. Beginning with the point of view. I write better from 1st person, because it allows me to get inside my protagonists mind.