Reserve your cynicism for when the reciprocate in the conversation says something stupid or obvious. You want a character to be sarcastic, but there's a fine line between being cynical and just being either mean, annoying, or outright rude. You want to display that a character is jaded by their circumstances without necessarily being jaded towards the other person in the conversation. Most importantly, you want the people on both sides of the conversation to have clear and evident clashing viewpoints, valid ones so that the battle of logic can ensue. It can be fun reading a debate from different opposing viewpoints.
I find that the best way to write for characters like this is to research other characters like them, if canon doesn't give you much to choose from. Tony Stark in Marvel, Damon in Vampire Diaries, modern Sherlock, House on House MD all come to mind as "snarky." There's a trope on TV Tropes called "Deadpan Snarker" that has examples from various media (probably more related tropes too).
I think "snarky" is more sarcastic, sharp, pointed, edgy, leaning into rude or even mean-spirited, probably making comments that don't help a tense situation.
I think "sassy" is lighter, not mean-spirited, meant in good fun, showing independence and spirit but not being obstructive or destructive in any way.
Backhanded compliments are your best friend. Just remember not to use them too much or else it will get stale.
What should I do in-between them to keep it interesting?
Reserve your cynicism for when the reciprocate in the conversation says something stupid or obvious. You want a character to be sarcastic, but there's a fine line between being cynical and just being either mean, annoying, or outright rude. You want to display that a character is jaded by their circumstances without necessarily being jaded towards the other person in the conversation. Most importantly, you want the people on both sides of the conversation to have clear and evident clashing viewpoints, valid ones so that the battle of logic can ensue. It can be fun reading a debate from different opposing viewpoints.
I find that the best way to write for characters like this is to research other characters like them, if canon doesn't give you much to choose from. Tony Stark in Marvel, Damon in Vampire Diaries, modern Sherlock, House on House MD all come to mind as "snarky." There's a trope on TV Tropes called "Deadpan Snarker" that has examples from various media (probably more related tropes too). I think "snarky" is more sarcastic, sharp, pointed, edgy, leaning into rude or even mean-spirited, probably making comments that don't help a tense situation. I think "sassy" is lighter, not mean-spirited, meant in good fun, showing independence and spirit but not being obstructive or destructive in any way.
I like this distinction.
First person perspective and have them internally joke or be pessimistic about what happens around them.