I was thinking about this post this evening and I do have a couple of suggestions.
First of all, don't be afraid to add mannerisms, gestures and other personal quirks to flesh out your teacher. One of the best things you can do is go to a coffee house, Internet cafe or similar location where people basically sit around, relaxed and either communicating casually with friends or finding ways to keep themselves busy if they're alone. Just people watch. Notice facial expressions, body language, how they use their hands, where the eyes focus. You'd be surprised at the strange mannerisms you can pick up this way.
Second, I watched the 10 parts of the ATAS: An Evening With Lost on YouTube, paying particular attention to Michael. It's a challenge since he's not on camera most of the videos. One thing I did pick up on during the first part is that when Damon and Carlton are talking, you can see Michael's hands resting on the table where he's sitting above and behind those guys. It felt very peculiar watching the minute movements of his hands. Almost too intimate in a way.
However, I did pick up on some interesting things. He sits in a very elegant way. Even when his elbows are on the table, he sits very erect. His arms and hands are draped casually, but still gracefully placed. I noticed he does something I've always done when sitting at a table, particularly when I'm interviewing someone. This is, leaning slightly to one side, at times one arm across the table, against his body, the other hand on chin or the side of his face. Otherwise, it's almost as if he's hugging himself with his arms formally crossed, hands draped over his forearms--if that makes sense.
One thing I saw in these vids I'd not noticed anywhere else: He has a very peculiar, one-handed way of twiddling his thumb. His hands are very long, slender, well-shaped. A surgeon or pianist's hands. His thumb is particularly flexible. He bends it into the palm of his hand, and then with the pad of each finger in turn, he circles his thumbnail very slowly; probably clockwise, but it was hard to tell precisely. You have to watch his hand very carefully to catch the motion. It's subtle, and like everything else about him, graceful.
Something that struck me that I'd only joked about before is that he has a cat's grace. He's very feline, and not merely his blue-green eyes and how he uses them to communicate, intimidate, emote, etc. His head is held high. His shoulders squared and back. At one point, he had his hand along the right side of his face and it made me think of Quintin Crisp.
I've been noticing more and more what a very small man he is. He's so magnetic and projects his presence with such authority, we often over look the fact that he is short and quite slender. The clothes tend to hide it well, particularly his baggy khaki's as Ben, and then the vests and three-piece suits he prefers in his private wardrobe. Many small men tend to fold in upon themselves, well aware of their limited stature and size. Michael is the exact opposite. He carries himself like an aristocrat. Almost Beau Brummel in many respects, right down to the umbrella he tends to have with him at all times. In Paris, he carried a full-sized one with a wooden hooked handle--very British, at least to an American's eye--instead of the collapsible black one that seems to be in nearly all the candid shots of him outdoors.
About his walk. Michael is a heavy heel striker, meaning he walks, shoulders back and his heels--and not the balls of his feet--hit the ground first, then he rolls onto the ball of his foot for the next stride. His stride is long for a man of his height as well. It's not a mincing stride as is also often seen in smaller men. He carries himself with a great deal of authority and control. He commands attention, despite his rather mild, meek personality.
His eyes. When he wears those lovely John Lennon wire frame glasses as Ben, he has his contacts in. That's why we're seeing photos of him and his eyes appear different colors. He wears aqua tinted contacts. His eyes are blue-green, but not nearly as vivid as the contacts make them seem. Since he does seem to suffer from a fairly pronounced nearsightedness, his real glasses have a thicker lens and refract the light differently. The Harry Potter framed ones in particularly make his eyes appear smaller. Those are his strongest prescription and if memory serves, he wears them mainly in the late afternoon and evening which might indicate a bit of nighttime impairment as well. He does have prescription sunglasses that are very dark. You'll see them in the pocket of his vest in most daytime photos.
With the nearsightedness, he will tend to squint when trying to focus at distance, most especially if he's wearing neither glasses, nor contact lenses.
Those are just a few of the more intimate details I can think of off the top of my head. I tend to pick up a ton of information when I look at someone. Not sure if that's a good thing or a weird thing!
Here's the link to the series of videos I mentioned:
www.youtube.com/watch?v=nCLFwicq1-YI hope this helps!