An important element of writing related to GMC is the Character Arc.
During the course of your story, at least one main character should grow, evolve, and/or change. This is defined as a Character Arc.
Does your hero distrust the police? Was he was framed by a crooked cop for a crime he didn't commit and recently got released from prison on a technicality? If the heroine is a detective, your hero has to learn to trust her or the romance can't develop. The path he takes to overcome his distrust is his character arc and his love for the heroine motivates him to learn to trust her.
Is your heroine shy, but she has to stand up for her learning disabled child against the school's uncooperative principal? Her character arc will involve educating herself about her rights and learning confidence in her ability to speak her mind about what she believes. Her child's well-being is her motivation behind this change.
Is your hero plagued by guilt over not being able to prevent his younger sister's drowning death when he was eight years old? Now his best friend and his wife have died, and the hero has been informed he has custody of their three-year-old daughter. The process of taking responsibility for the child and healing from his past experience are a large part of his character arc.
Every character has at least one weakness, even if he's a claymore-wielding Scottish laird or a high-profile district attorney with great legs and perfect hair. No flaws equals not human, and readers want to be able to relate to and sympathize with your hero and heroine. Help them move on, heal, learn, grow, etc. to become more confident and accepting of who they are.
Isn't that what we all work toward in the real world? :)
Next week--Author Intrusion!
Romance...With A Kick!