When a film shows a countdown, the audience receives an explicit measure of time. The cuts around it can make us watch that measure with unusual attention.
Compare two appearances of the clock
Record the displayed time at two moments and describe what happens between them. Do not assume the sequence is playing in continuous real time. A film can compress, overlap or omit actions while returning repeatedly to the same numerical reference.
Listen for changes in sound as well. A repeated tone, a brief silence or a burst of dialogue may redirect attention from the digits to the people trying to act.
Notice who can see the timer
The viewer may know the remaining time while a character does not. In another scene, both may share that knowledge. These arrangements create different expectations even before the countdown reaches its end.
For a short analysis, choose one cut away from the timer. Explain what information that cut supplies and what it makes you wait to learn. You do not need to audit every second to discuss the scene effectively. The more interesting question is often how the film alternates between measurable time and the experience of waiting.