There are two schools of thought on what is more important, plot or character; you need to decide before plotting which school you like.
The character-first school believes that character is important because it’s the choices of the characters that define the story. The major advantage is that avoids “Acting Appropriately Stupid”, that requirement where the only to further the plot is if a character or three makes a stupid decision that is against their character (such as splitting up and taking showers while the psycho killer is a known factor or ignoring their intuition which has served them well). Also, it allows for plumbing character depth, but usually at the cost of a coherent story; the plumbing usually presents so many side trips that the story is ignored in order to better explore the character.
Admittedly, I’m for anything that avoids the AAS issue, but the character-first story has a problem with it: Little actually happens. The writer usually gets so wrapped up in the character, that he is afraid to affect the character long-term; the character is considered sacrosanct, and is thus not allowed to change. After all, if what the writer likes about the character changes, then it’s no longer the same character and will no longer be as fun to play with. However, a static character is only so fun to read in the long term; people want to see some change in the character over the long term.
Worse, it becomes harder to avoid the Mary Sue problem (a “Mary Sue” is a character, usually female (males are “Gary Stu”) that is perfect in every way, and has abilities that far outstrip any competition, and has a “destiny”, but has some dark history that or desire that is counter to her plans). Instead of a realistic character, the character ends up gaining more abilities and sending her further away from any kind of reality in order to make some sort of change in the character. This further alienates the reader, as he can no longer relate to the character (she may have a dark secret (she may have crush on the bad guy, and hopes to reform him, but she can defeat anyone, has a really cool magical pet, and has a lot of really nifty abilities, and she can’t do anything wrong except as required by plot). Could you relate to her?
By going with the plot-first approach, the character is forced to change, and in a realistic way. Also, the characters won’t drag the story down, as you won’t explore the side trips; you’ll focus on the plot, and that’s a good thing. Your characters will also change as they adapt to the plot, and react to it realistically. Also, it allows you kill characters and not feel guilty about it. By making the plot more important than the characters, you can also throw stuff at the characters and see how they react to it.
That is, you can explore the characters’ characters in ways that also move the plot along, and that way you can have fun with your characters. As long as you can avoid the AAS syndrome, you can actually do what you want (explore the characters), while doing what you story needs (the plot). By reaching that compromise, you can build a stronger plot, and have fun doing it!