Observing other people will teach you a lot about the people you are observing. The more you observe the same person (or people), the more you'll learn about them. Looking from afar means you can think about what you are witnessing, whereas when you're participating, your mind is more focused on what to say and do, rather than fully grasp what the other is doing.
For this reason, when your MC interacts with someone, the readers get to feel how the MC reacts to the exchange; but when the MC is observing someone, they can notice things that couldn't have been observed during an exchange. Especially, if the MC is observing someone who isn't aware they're being observed. It's very helpful to flesh out the observed one.
The MC could be at the Prom having fun with their friends, noticing very little besides their own fun. Then the MC has to use the restroom and, on their way, they walk by a fellow student in the corridor who is popping his knuckles. The MC wouldn't have paid him any attention if it hadn't been for that irritating noise, but as it was the MC looked back and noticed his grim expression.
The MC recognised him, though. Not by name, but he'd seen him being bullied by Mark Jones and his friends. Poor guy must have been their target tonight again, and was hiding in the corridor. On his way to the restroom, the MC kept remembering his figure. It was weird. He usually had this 'please help me' expression, while tonight... he seemed so fierce! Had he tapped into that spirit earlier in the school year, Mark and his cronies might not have targeted him as much.
The MC comes across Julie shortly after. On the return, they come together, chatting all the way through the empty corridor, chatting about Peter, who got caught trying to pour some vodka into the punch.
Forgive the clichés, but that's the general spirit. With one observation, you get to see a side of the bullied boy that no one has noticed, and then the moment is gone and the character is apparently forgotten. Moreover, we have no idea if the MC's thoughts on what must have happened are real or not. For all we know, the boy might have been about to take a page out of Carrie's book.
AS an introvert myself, I'm used to sitting back at a party and watching the people around me. Not for long periods of time (I don't want to look like an outcast), but in between conversations I get a glass of juice and a piece of cake, then give them my undivided attention... while analysing the kids' games, how their parents are involved in a heated conversation and yet never stop throwing an eye at their shenanigans. Those little things. And it has happened that, while discretly observing, I find myself observed by someone. Our eyes meet, we each recognise what the other was doing but disguise it by saying something along the lines of 'I've been meaning to ask you about...". That way, you weren't 'observing', you were simply looking at the person you wanted to talk to waiting for the right moment to start a conversation you've been dying to start.
Besides, well done, a fleeting scene can be more effective in fleshing a character than three long chapters cobbled around them.