Those characters you're writing into your story are important and they're very easily annoyed when you direct them or force them to do something that's against their nature. Sure, you're the author and they will do what you want them to, but consider that they are the connection point between your story and your reader. Trust me, if your character does something odd because you told him to, your reader's going to toss your book across the room and find something else to read.
Now, there are numerous ways to do this effectively. I used to plot out a novel idea, followed by plotting out my characters' personalties, so I would know what they would do when it became necessary, but I discovered that they have their own style of consciousness and they will not back down or give up when you make a mistake with them. I was writing a fantasy story once where a lead character leapt from a cliff face to latch onto a dragon's head and eventually kill it, only to ride the doomed beast out of the sky to his own death. I wrote the scene, all happy that it was a crowning moment of sacrifice for the character.
My character said, "Um, no. I won't jump from a cliff when I don't have to. I'm crazy, but not stupid. You'll have to rework that."
That refusal took me almost two weeks to figure out. It turned into grabbing the dragon's dragging foot, followed by a long, tortuous climb up the body and neck, culminating with strikes at the right time to guide the beast close to the mountain where the character could bravely jump off again before the long dive downward. The scene tuned out far better.
My character said, "See? Now, after all that work, can I get a pretty elf to hang with?"
The Philistine
Since that point, I only have a basic knowledge of the type of personality each character has and I try to angle for it on a consistent basis. Every now and then I still get a scene where the characters are kind of wooden and clunky. I read over what I wrote and listen for my characters' voices. They tell me the story, but they also keep me involved with their place in the story.