I mean in the Big 5 personality model sense. Someone who is neurotic is sensitive, more prone to negative emotions including fear. Someone who is low in neuroticism is less fearful and sensitive on average. So for example you could put two people in the situation of being about to jump out of a plane and one would feel terrified for their life while the other would feel only mild apprehension.

EDIT: People are a complex blends of emotions and motivations, so you're certainly right that can be another explanation for the same observable behavior. I really liked the comment about adrenaline junkies too. My point was only that on average it's low-sensitivity people who engage in those sorts of activities. Scaredy cats like myself stay home and read a good book.