Writing: playful banter between two characters

Someone posted the question: “Where is the line between playful teasing and bullying when it comes to sibling characters?”

I replied:

On your question.
Keywords for playful bantering: boundaries, mutual respect, safe space, balance, performance/play/creativity.

Boundaries, safe space & mutual respect:
Both know how far they can go and where to stop. Both will go as far as they can, but also stop when they are about to step over the edge where it will start to hurt / where it is no longer safe. Both strife to keep the bantering a safe space.

How to show this: One will step over the edge, see the hurt and stop immediately, will give a signal that they understood this was too much.
One will step over the edge. The other might respond with even harsher response to indicate “you went too far”. Both might be silence for a moment, acknowledging this was too far. “You went too far” might also be something like a punch on the arm.

Balance:
One will always respond. Both will get equal space for expression. Responses are at least as terrible. (Depending on the form of the game) There is always a form of funny.
Both get the opportunity to score. Both challenge the other to score and might give deliberate hints and openings to where the other might even top the previous remark, like a constant buildup. Both create opportunities for the other to score. Both win. Both will create deliberate opportunities for the other to win.

Disbalance 1: one that started will try to get last word. Continue to push. Maybe they are angry. Maybe something is frustrated or wrong. The other will call the other out. “You need to stop. What the fuck is going on. Are you OK?”

Disbalance 2: One is harsh / mean, and expects the other to just take it, but becomes angry when the other responds in a similar way.

Disbalance 3: the other creates no openings, will be defensive or offensive without any opening for a response. The other will create mostly opportunities to win for themselves.

Performance/play/creative:
Both know it is a play / playful performance and that creativity might be and important part of that play. This play has rules. And those rules can be complicated or simple. (‘Complicated’ can be: “each sentence or each 3rd word in a response has to be/contain a swearword” and/or: “each swearword is related to an animal” like “those shoes are as attractive as a duck’s anus” and/or “each next response has to be worse / more absurd”) Those rules can be different each time and will probably be “what we come up with at that moment”. Part of those rules are balance, boundaries and mutual respect. Complete absurdity might be part of it.
This play can be performative to others, either to change the dynamics in a room for the better (think of improvised stand-up comedy, but performed as a play between 2 people, where the audience is played as well, but not directly addressed), or to deliberately annoy other people.

Respectful performance: the boundaries of the audience are respected. The audience is observed and the moment the audience responds negative / gives signs that they no longer feel safe, the play changes so things shift back to a safer place.

Bullying:
One-sided. No giving. Only taking. No balance. No respect. Not a play. No respect for boundaries. No respect for the other(s). Aimed at winning at the cost of the other.