I was trying to "restart" a character of mine some time ago, because critique partners had told me he didn't come across as having a very strong personality. This character is a particular challenge because he's got a degree of "strong, silent type" about him. One would be tempted to put him first into a situation where he really wasn't interacting with other people, just because he's so comfortable not saying anything. I've read enough Facebook memes about introverts lately that I'm sure I'll have readers who think I shouldn't put him through it.
But I'll still argue that putting this character into personal interaction is the best way to introduce him.
I'm in deep point of view here, and so my readers get to share his internalizations. Thus, if someone is interacting with him and he doesn't feel comfortable with it, they'll know, because his discomfort will be evident. The other thing I find I can do is put him in interaction with someone he does feel comfortable with, and show how he doesn't always make use of an opportunity to comment. How he judges the interaction styles of other people. And what kind of thing would be so unusual that it would actually move him to speak to a group (he gets rather upset about the behavior of some younger students).
My other character is less quiet and hangs around with three of his friends constantly. I chose to start him in a moment of reflection, to give a bit of his mood and backstory, but thereafter I put him straight into interactions with his three friends, to show what their social roles are, and their styles of interaction.
Which is to say that if you're introducing a character, try to put them into interaction with other people when possible. If you've got a wanderer who never sees a living soul, that's a bit different - but chances are the story will still bring that person out of their comfort zone and into interaction with other human beings. Show us those interactions. It will show us so much about your character.