Characters.

Characters can be like best friends, or best enemies. They can be helpful and harmful to your story. Knowing a character is inevitable, and is going to move your writing along faster the sooner you find out who these characters truly are. Knowing mundane details about your character not only builds your personal appreciation of these people that are going to be a large part of you life, but also makes them more interesting for your readers and may adjust the storyline because of the characters particular likes/dislikes/circumstances/you get the picture.
Example:
Your main character is a pirate.
Is he/she a parrot pirate or a monkey pirate?
If you don't know the difference you should go back to pirate school.
Knowing which he/she is leads the character into a different direction and makes them more interesting than a pirate whose personal preference on an animal sidekick is not known.

When I write characters I tend to base them on who I think people should be with a little tiny bit of real people. Especially when creating heroes. Heroes get to be larger than life, so I make them into people that I think they'd be, not necessarily based on people around me that aren't heroes and whom heroes shouldn't be based on. While your characters should have realistic abilities/wants/desires/(you get the picture), they also can exceed the limitations of the people you see/know.

Also, knowing the back story of your characters helps your story be more complete and can offer that extra fluff to make a 44,000 word story into a 55,000 word story (I know from experience). I enjoy using several different "character sketches" to build on my character's back stories. These usually entail writing out details about birth dates, likes/dislikes, family history, physical descriptions, wants/fears and so on and so forth. If you Google "character sketch" you probably can get a lot of sites with stuff you can use to plot out your characters personalities and histories. I want to say though, that not only does knowing your characters help you enhance your story, it also leaves your reader more satisfied. It's like the difference between a plain bologna sandwich and a bologna sandwich with ham, salami, mayo, lettuce, tomato, pickles, onions and mustard.

While you shouldn't leave you readers with questions, you also shouldn't tell them too much. A reader doesn't need to know how many sheets of toilet paper your character prefers to use, unless it matters (later in the story they run out of toilet paper and a mutiny on the ship causes the main character to go rouge).

I feel like this post is very scattered, so I may touch on this subject again in the future.