Most of the time, what I end up seeing are characters saying things that, if this were played out in the real world, they would never say. I see characters having words coming out of their mouths that given their personalities, they would never say. In many ways, it is like, Baby Herman. You know the guy, the little darling in Roger Rabbit who happens to be a chain smoking, gambling, foul mouthed baby...
I don't care if you have a great dialogue sequence you put together with great lines, if the characters wouldn't say this stuff, then they shouldn't. And yes, this goes for the internal dialogue they have with themself.
I could go on and on with situations and people, but I think you have the idea. What you need to do right now is to really examine your characters. Not what they say but who they are. Once you get a good grasp on this, then go back and look at what they say. Is this right? While you are it, also look at where and when that would happen. Would that conversation really happen in church? In the board room?
And no excuses for the story being fiction or "fantasy." Your characters have to act and behave like humans for us to believe your story.
Have a great day!