how would the meaning of the sentence change if tremulous were replaced with sarcastic or with nonchalant?

If you describe someone as nonchalant, you mean that they appear not to worry or care about things and that they seem very calm.

For example: Clark's mother is nonchalant about her role in her son's latest work.

Denis tried to look nonchalant and uninterested.

But tremulous is something that is caused by nervousness or shakiness.

So instead of someone being nervous they would be very calm.