The main reason authors do this is to add drama and suspense. It builds anticipation and makes the reader emotionally invested in the outcome. Also, tension helps drive the plot forward and makes the story more memorable.
Authors use tension to keep readers engaged and on the edge of their seats. It makes the story more exciting and compelling.
Another way is through the slow development of the relationship. The captor may start off as cold and distant, but as they interact more with the captive, small changes occur. This slow burn while the captive is still in a kidnapped state keeps the reader on edge, wondering when and how the relationship will fully develop into a romantic one.
Authors often create tension through the initial reluctance of the characters. For example, if one character is against the idea of an arranged marriage, there will be conflict. Another way is by introducing external factors like family expectations or cultural differences. This can put pressure on the relationship and create tension.
Another way is by showing the desperation of the characters trying to rescue the kidnapped one. If they face numerous obstacles like powerful enemies or difficult terrains, it adds to the tension. For instance, if the Cullens have to fight through an army of new vampires to reach the kidnapped character, the readers will be worried about their success.
One way is through the power dynamics. For example, if the male lead is a powerful mob figure and the female lead is an ordinary person, the difference in their status creates immediate tension. He has the power to control her situation, which makes their relationship precarious.
By introducing dangerous enemies. In these fanfictions, there could be new, more menacing villains than in the original series. The threat these villains pose to Harry and Hermione, and their plans, keeps the readers on the edge of their seats.
By limiting the time. If a Pokemon has only a short amount of time to be saved, it creates immediate tension. For example, if a Pokemon is poisoned and will die in an hour if not treated.
The first paragraphs often introduce a problem or a conflict that immediately grabs the reader's attention and builds tension.
One way to create tension is to have characters with conflicting goals or interests. This creates internal and external conflicts that keep the reader on edge. Another approach is to use foreshadowing to hint at something bad that might happen, making the reader anticipate and worry.
You can create tension by setting up deadlines or time limits for the characters. Like, they have to complete a task before a certain time or something bad will happen.