The sentence describes the actor's behavior when his minor desire ("slightest whim") was not satisfied.
- Action: The actor became ______ and started "snapping even at those not involved in the filming."
- Context: A failure to meet a minor, perhaps unreasonable, demand led to an aggressive, irritable, and childish display of temper.

The word in the blank must describe a state of being irritable, grumpy, or bad-tempered, especially due to a petty annoyance.
- A. grim: Unrelenting or forbidding in manner. While negative, it refers more to determination or seriousness, not pettiness or snapping.
- B. fastidious: Very attentive to and concerned about accuracy and detail. This describes precision, not temper.
- C. querulous: Complaining in a petulant or whining manner. This is a perfect fit for someone who is irritable and prone to snapping over minor issues.
- D. sullen: Bad-tempered and sulky; gloomy. This implies silent resentment, but "snapping" suggests active, verbal complaining.
- E. chivalrous: Courteous and protective, especially toward women. This is the opposite of the actor's behavior.
- F. petulant: (Of a person or their manner) childishly sulky or bad-tempered. This is a perfect fit, matching the description of someone whose minor desires are not met, leading to an irritable reaction ("snapping").

The two words that best describe being childishly ill-tempered and prone to complaining are querulous and petulant.

The correct choices are $\(\mathbf{C}\)$. querulous and $\(\mathbf{F}\)$. petulant.