In Chapter 3, Steinbeck provides the reader with some of Cyrus Trask's backstory. The following excerpt includes an important idiom that shapes readers' understanding of Cyrus as a character:
Cyrus Trask did have his five minutes of horror while they cut the shreds away and sawed the bone off square and burned the open flesh.
The phrase "five minutes of horror" is a play on the idiom "five seconds of fame." The original idiom is typically used to describe a situation in which a fame- or attention-hungry individual gets their short stint in the limelight. Switching "fame" out for "horror" elucidates the grotesque association between these two concepts.
This passage, in conjunction with many others, really helps to meaningfully characterize Cyrus Trask: he is a man who craves attention, respect, and prestige, but cannot withstand the pressure and sacrifice that would get him to that milestone organically. He gets his "five minutes of horror," but he leaves the military immediately after. Once Cyrus arrives home, he starts lying and doesn't ever stop, so much so that he soon forgets himself what the actual truth is. Cyrus's lies are so integrated into his person that Adam doesn't believe they aren't real.