First, find out what your hero wants, then just follow him, according to legendary sci-fi writer Ray Bradbury who, to be honest, makes it sound all too easy, but perhaps he does help to resolve the literary chicken and egg question of what comes first – plot or character.
If you come up with an ingenious plot and plonk in any old protagonist to service it, my hunch is that the end result will be two-dimensional and emotionally unsatisfying. If you start by creating a character, building her wants and needs, her hopes and fears layer by layer then you will be able to devise a plot that tests her strengths and exposes her weakness. If you know in advance what your heroine’s heart’s desire is, then you will be able to thwart her and subsequently to discover how far she is prepared to go to achieve it. This is the difference between a linear plot: a) happened, then b), which led to c) and one which is more organic: a) happened, which had this effect on your heroine making her feel something that caused her behaviour to change thereby influencing your hero, which provoked him to do such and such. Putting character first enables you to explore the relationship between psychology and action and ultimately to look in a fuller sense at cause and effect.
So if somebody asks you the literary chicken and egg question, the answer is definitely chicken – it’s character that should come first.