A strong vision can be the central idea of the character, and will dictate the function and playstyle of the character. If you are envisioning a deadly assassin, then it suggests that the character's function is Damage, keeping alive via Aggro Management. He may employ simply his most basic but deadly sneak attacks and thus will not rely on too much active involvement. The inspiration for the flavor can be historical or literary figures, or alternatively, certain in-game mechanics: for example, shield wielding can be a defining feature of your toon.
One of the typical ways flavor comes into play is in choosing your race. If you have a strong preference for a certain race - congratulations, you have one less thing to worry about!
Suddenly: Kungfu.
This needs to kept in mind because it will influence later decisions too.
For many, however, flavor is unimportant, and is not a significant factor.
Archibalde was imagined to be a monk defended by the faith he has in his god. His convictions force him to stand up against the strongest enemies.