Characters are really the central element of any novel. These are the people we follow on their journey. We laugh with them. We cry with them. We feel with them. The problem, however, is that writers are often so obsessed with "the story they want to tell" and "the plot" that they forget the characters. Instead of the characters being someone the readers can connect with, they become two dimensional plot devices, or even worse, unrealistic characters.
It doesn't matter what the genre is that you are writing, the characters have to be real. The words that come out of their mouth, the way they react to situations, and the way they interact with other characters has to be believable and true. Characters who are not in alignment with their dialogue and behaviors become a real scar on the book.
Let me give you a couple that I see far too often, and let's begin with those romantic suspense novels. A common plot we see is the heroine running from an abusive relationship. First of all, this is far too common of a plot line and is a bit cliche. Still, it is a common one. But here is where the problems show up. If someone is truly running from an abusive relationship, the thought of starting up a romantic relationship with this new stranger that showed up in their lives back in Chapter 2 or 3 is just not believable. A relationship got them into the problem that is the basis of your conflict and a new relationship is going to be the furthest from their mind.
Try this one. Historical novels are notorious for the heroine to end up in an arranged marriage. OK, we get that part and we can work with this. However, when they are now shipped off with the new husband they are not likely to go skipping away overly happy. Look, they are essentially being kicked out of their house into one of a stranger. Think how you would feel.
Last one... we see these in contemporary novels. A character is in a financially bad situation but decides that now is the time to take a cruise with a friend? Really? I don't know about you, but when finances are running low, most people will not think about going out and spending more money.
The key with most of these situations is that the author is not thinking. They have to put themselves into that situation and think truthfully and honestly how they would feel in that situation. What would they say? How would they act? Quit thinking about how you just have to get the character to that next scene, or how you want that sentence to sound. Think realism here.
A good practice for this is to think about all of the stories you have read (including your own) and you find something is just not sounding right. Go back and see what the characters are saying or doing. My bet, is you will find that lack of realism. Fix that and you will be in great shape.