To create dramatic irony in this detective story, the writer could utilize the technique of revealing the killer’s identity to readers right before the detective unknowingly enters a scene with the killer. This approach heightens tension because the audience becomes acutely aware of the danger the detective is unaware of, creating a sense of urgency and anticipation.
For example, as the detective pieces together clues from the cold case, the narrative could alternate between her investigations and scenes from the perspective of the true killer. The reader might see this character interacting with the librarian in seemingly innocent ways, perhaps even making light-hearted jokes or observations about the library's history. Meanwhile, the detective might be assembling evidence that points away from the librarian, fostering a sense of security.
As the detective makes progress and begins to suspect her romantic interest is an innocent ally, the reader knows that the true danger lies within the unassuming patron who frequents the library. When the detective finally decides to confront the alleged 'suspect' or investigates a lead related to the recent murder, the suspense will be palpable, as the audience braces for the inevitable moment she unknowingly walks into the lion's den.
This technique not only develops tension but also deepens engagement, keeping readers on edge as they plead internally for the detective to connect the dots before it’s too late, thus enhancing the overall dramatic irony of the unfolding story.