Once you create your characters, giving them flaws and strengths and back stories and personalities that are real, they essentially take on a life of their own. It's a strange concept to readers, I imagine, but if the author then tries to make these characters blithely say and do things in opposition to who those characters are (just because its convenient to the author for this to happen) it will ring false. Jarring the readers' expectations in this way is going to pull them out of the story. Yes--your characters can make mistakes. They can be forced by circumstances to do something they don't want to do (great inner conflict!) Your heroine can dissemble, or do something recklessly...even lie, if the reader sees strong motivation behind it. But dropping in language or actions that are totally out of character, and for no good reason, will make her far less real. How many books have you tossed aside after becoming irritated with the main characters for being "too dumb to live" or for coming across like cardboard?
Back to planning out a story....how much do you plan ahead? Do you sit down and just write (seat-of-the-pants), or are you a plotter, who plans everything carefully first? Something in-between? I try to work out a very simple "Plot Spine" that shows me where I think I will end up....and try to list the key, progressive turning points between the beginning and the end. As I mentioned above, things may totally change. But...it can help me see where I'm headed! A simple road map means less chance for time-consuming detours and false turns.
Consider trying this. For the