MERCURIAL = (adjective) 1. containing or being somehow related to mercury; 2. having a temperamental, active, or unpredictable personality; 3. quick, lively, and shrewd, like the god Mercury

Synonyms:erratic, changeable, fickle, volatile, unpredictable, fluctuating, temperamental, moody, lively, quick

Antonyms:constant, invariable, steady, stable, even

Examples:
- I wanted a mercurial thermometer but I got a digital one.
- The comedian relied on his mercurial wit, not rehearsed jokes, to entertain his audience.
-Lisa is a bit unpredictable with her mercurial mood swings.
-His mood is somercurialthat we never know how he?s going to react to anything
NONCHALANT = (adjective) casual and unconcerned

Synonyms:casual, blasé, indifferent, unconcerned, unflappable, apathetic

Antonyms:worried, concerned

Examples:
- Surprisingly, when John was told he won the contest, he was very nonchalant, and didn’t seem that excited.
- Even though Marc was nervous when he asked her out, he tried to appear nonchalant about it.