Every time I start a new project; whether it's a mystery game or a manuscript, I try to think of the story as a whole; or, if you wish, as a large pot of stew.
First you need a plan. It's the same with writing; first you need your plot - if you're writing a mystery (like me) you need to know at least the identity of that first body. Now, who did it might come later, but if you're like me you don't want to get too far before your murderer unmasks, even though it's just to you at this stage.
Now that you've got your plot (or pot) you need your ingredients; your characters. Some are the main characters - your meat. There's nothing stopping you creating a stew with just meat, but that's not really a stew. You need your vegetables, those big chunks of potatoes, carrots, turnips and celery that add flavour and colour. Every plot needs those colourful supporting characters as well; those wonderful personalities who might be a little too wild to build an entire plot around, but whose quirks add a unique zest to your story.
Just as when you're concocting a stew you always add your spices - your garlic, salt, pepper (and maybe a dash of oregano or thyme) remember to sprinkle some seasoning to your plot. Dialogue is vital, of course, but characters do far more than talk. Remember to let your readers experience what your characters are experiencing. Put them in the room. Let them see what your character sees, let them smell the fresh coffee or the lingering scent of a fire. Let them run their hands over the worn velvet of Grandma's drapes, taste the moistness of a carrot cake or hear the scrabble of mice in the ceiling.
Of course, stew isn't made in an hour and neither is a book. Simmering a stew allows the ingredients to cook and all your flavours to mingle. Let your plot bubble. Go back. Adjust the spices, maybe add a few more carrots. You've got time. But remember, sooner or later, that stew needs to be served.
Think of it as a deadline.