There are a number of elements and techniques authors use to create suspense. One Finney uses is characterization. Another is chronological detail; this incorporates flashbacks to near-past events and flash-forwards to potential, imminent future events.
Finney's early characterization establishes Tom as a loving, though ambitious, husband of a lovely pleasant wife whom he admires and enjoys. There is no discord between then except that Finney casts her as the voice of reasonableness and truth when she points out Tom works too much and too hard; even her name, "Clare" is spelled as a scramble of "clear" (instead of "Claire"), making her a symbol of clear thinking:
glancing at the desk across the living room, [Clare] said, "You work too much, though, Tom--and too hard."
Later Jack's characterization develops him as a hard-working, insightful, practical, reasoning man. This dramatizes his flaw, which is his ambition. Suspense--already being developed--is heightened when we follow the progression of his thoughts and rationalizations when he watches the yellow sheet of paper blow along the ledge below him. When Tom finally leaps to a choice of action, as opposed to coming to a choice logically, we are suspensefully captured in the event and the potentialities within the momentous decision. Since we want Tom to be sensible and safe, we feel intense suspense knowing he will not be.
When Tom is on the ledge, the key suspense creating device is the detailed chronological progression the narrator gives that takes us step-by-step with Tom. This is augmented by flashbacks of Tom's life that give explanations of what is on the yellow paper and why it is so important. Flash-forwards to what Tom will experience if he loses his courage or balance and falls to Lexington Avenue is a key technique that creates intensifying suspense as we envision Tom's potential death with him.
He saw himself stumbling suddenly sideways as he crept along the ledge and saw his upper body arc outward, arms flailing. ... [He] felt his balance leaving him. He saw himself falling with a terrible speed as his body revolved in the air,