I have a habit of looking at RP as an acting exercise over a writing exercise, because that's where my background lies. (I've never considered myself much of a writer, but that's just me.) And one of the concepts that's really important in a performance, and this kind of echoes one of Arctique's points, is respecting the shape and concept of the character you are interacting with. It's like professional wrestling. You've got to sell what they're giving you, or else every interaction feels the same.
An RP interaction is structurally understood to me as a game of catch. Someone throws you a ball, and you receive it, and it's good propriety to throw the ball back, right? If all you're doing is catching balls, but not returning them, then the exchange can be one sided. So I use this combination of ideas to create a scenario where I am respecting the concept of who I am interacting with, and also making sure my responses give them plenty to work with, when at all possible. Granted, casual banter RP can be a little light for this, but as a general rule for this, I think it's important to sell what someone's giving you, (within reason), and never leave someone hanging in an interaction. In longform RP, I try to always give at least 3 things for my RP partner to respond or react to, which is maybe too strict a mechanical mindset, but it helps for me to structure it that way.
At the end of the day, I think it's fun to really react strongly to people's characters, because it gives your own character a variety of emotions in a variety of interactions, and this bitch loves emotional spectrums.