Slowly. The hero has to be patient. He can't rush the skittish heroine into anything. He might start by being just a friend, gradually earning her trust.
A skittish heroine in a romance novel is often unpredictable. She might be hesitant to fully commit to a relationship. For example, she could shy away from the hero's advances at first. This adds an element of mystery and suspense to the story. It also allows for character growth as she overcomes her skittishness.
The hero might start doing small acts of kindness for the heroine, even if she initially rejects them. Over time, these acts can wear down her defenses. For example, if the heroine loves books and the hero starts leaving rare books for her at her doorstep anonymously. Eventually, she'll start to wonder who is behind it and might soften towards the hero.
Often, at first there is a lot of animosity. The heroine is understandably angry and scared due to being kidnapped. But as the hero shows some kindness or vulnerability, the heroine begins to see him in a different light. For example, he might protect her from other threats while she is in captivity.
One way is to give her a tragic past. For example, if she has been betrayed or lost someone dear in a cruel way, it could lead her to close off her emotions. This past trauma can be a driving force that makes her reluctant to show or feel emotions in the romantic context of the novel.
The development can be aided by the heroine's own growth. As she gets to know the hero better, she might realize that his dominance is not all bad. She could start to see it as a form of strength that can be beneficial for them both. And if the hero respects her boundaries and is willing to step back when she's really scared, it can lead to a more positive relationship.
Most of the time, the hero/heroine is initially repelled by the villain's actions or nature. But then, the villain may show a different side, perhaps a more gentle or understanding side in private. This creates a sort of internal struggle for the hero/heroine. In 'Poison Study' by Maria V. Snyder, Valek is a rather intimidating and morally ambiguous character. Yelena has to work with him and over time, she discovers his loyalty and other positive traits, and their relationship evolves into a romantic one as she peels back the layers of his 'villainous' exterior.
Challenges also play a role in developing this dynamic. External factors like family opposition or misunderstandings can test the hero's adoration. But if he overcomes these challenges, it shows the depth of his love for the heroine. For example, if the hero's family doesn't approve of the heroine because of her lower social status, the hero has to fight for their relationship, which in turn strengthens his adoration for her.
Often, it starts with a professional relationship. The nanny is hired to take care of the kids or do household work. As she spends time in the house, she and the male lead start to notice each other. Small interactions, like sharing a meal or talking about the kids, lead to a growing interest.
Through communication. If they start talking, the heroine may realize that the hero is not as scary as she thought. For example, the hero could share his own fears and dreams, which would make him more human in the heroine's eyes.
In such a romance novel, the plot often becomes more passionate. The heroine's condition could be a catalyst for deepening the connection with the hero or could introduce unexpected rivals or obstacles to their love story.