Both Uses

crucial
in
Ulysses, by James Joyce
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Crucial moment.†
Chpt 15 *

crucial = very important
On the other hand what incensed him more inwardly was the blatant jokes of the cabman and so on who passed it all off as a jest, laughing 1530 immoderately, pretending to understand everything, the why and the wherefore, and in reality not knowing their own minds, it being a case for the two parties themselves unless it ensued that the legitimate husband happened to be a party to it owing to some anonymous letter from the usual boy Jones, who happened to come across them at the crucial moment in a loving position locked in one another's arms, drawing attention to their illicit proceedings and leading up to a domestic rumpus and the erring fair one begging forgiveness of her lord and master u†
Chpt 16

Definitions:

(1) (crucial)
very important or necessary -- often because it determines how something else will turn out
(2) (meaning too rare to warrant focus)