Final answer:
The word 'nonchalance' fits best in the given sentence as it represents calmness and composure, akin to movie heroes who are unaffected by stress.
Step-by-step explanation:
The correct word to use in the given sentence is 'nonchalance.' The sentence describes a state of being unfazed or cool, which is what nonchalance means. The phrase 'as cool as a cucumber' is an idiom that denotes being calm and composed, even in stressful situations. Movie heroes are often portrayed as being very calm and collected, hence why 'nonchalance' is the word that fits the context best. Thus, it's the apt word choice, aligning seamlessly with the intended portrayal of an individual maintaining a serene and unbothered aura, echoing the resilient coolness emblematic of heroic characters in cinematic narratives.