Let's assume that you have drafted an outline of your story and feel good about the way the basic story has been devised.
Before you even start filling in the detail within that outline, you need to give some serious thought to your characters' motivations, strengths and weaknesses and the way in which they will impact on the story.
If you don't give enough advance attention to this aspect, and if you don't dig deeply into what makes your character's tick, then they are likely to end up as two-dimensional cardboard figures with no life to them.
Of course this is a recipe for disaster.
However, if you take the time to analyse each major character in some detail in advance, you will not only ensure that their characteristics fit the plot and become believable, but you will also find that as you build them, certain traits will emerge which could trigger more creative elements within your plot.
Many people use a character prompt sheet for each character, with general identity answers as well as other searching questions and answers about their make-up.
Whilst this is usually a very comprehensive document, I am only going to consider a few of the more important aspects.

• What are the character's inner needs? These will help to determine the general make-up of his behaviour pattern as well as give an indication of what motivates him.
  These core needs can include things like the need for excellence in what he does; the need to belong; the need to dominate others; the need for excitement; the need to always be seen as a winner, and so on.
  Any one of these could put a different slant on a relevant scene within the story.
  
• What happens when a character is denied any of these needs? How does the stress resulting from this denial affect his behaviour? Indulge in this self conflict and imagine how his reaction to this denial can create more interest in the relevant scene in the story.
  If you have built your character to be a reactive man, allow him to react spontaneously to any situation that denies his core needs.
  
• What is the character's sexuality? This could affect his attitude towards the opposite sex, or to particular types of persons within the opposite sex.
  If he has suffered a bad experience which he perceives not to be of his making and he feels ashamed or angry, leverage this internal anger into the story-line.
  
• Is there any single event in the character's past life that had a significant impact on him? Bear in mind that if there is such an event, it will likely affect his personality and motives for being the person he is.
  This could be used in a novel using back-flashes as the character is being developed in the story-line.
  It could even be the main thrust of the story, with events leading up to a solution to the problem.
  

Believable characters are very important to your story.
They are the perpetrators of conflict.
And the more conflict you have in your plot, the more tense your story will become.