In "The Tell-Tale Heart," the author builds up tension during the rising action in a few ways. One way is through the narrator's increasing obsession and paranoia. As the story progresses, the narrator becomes more and more fixated on the old man's eye, which adds to the sense of unease and suspense. Additionally, the author uses vivid and descriptive language to create a creepy atmosphere, making the reader feel on edge. The pacing of the story also contributes to the tension, as the events unfold slowly and deliberately. These techniques work together to keep the reader engaged and anxious as the story builds towards its climax.