It seems like Aeolus is the easiest face to read. If his mouth is open in shock, he has a Poobah, if he's smiling and you can see his teeth, he has a Fizbin. If he's slightly smiling and you can't see his teeth, he has a Mimi. If he has a bad hand, he will frown. He doesn't have a very good Poobah face. To be fair, he does say he's not good at the game.

Katie is a good bluffer. Her facial expressions are usually the opposite of her hand. If she looks mad and she hasn't folded yet, that means she has a Poobah. If her face is neutral, it means she has a Fizbin. Generally, when she's smiling, it means you don't have to worry about her.

Sosi smiles with his eyes open when he has a Fizbin. He smiles wide with his eyes closed if he has a Poobah. His facial expression is neutral if he has anything else. He almost always bets, even when he has a bad hand. Probably because he's rich and can afford to lose money.

Esme's face is usually neutral, but when she has a good hand she has an easy-to-see smile.

Joel's face is usually neutral, but he has a subtle smile on his face when he has a Poobah.

I'm not sure Thomas's facial expression ever changes. Either I'm missing something or Thomas has mastered the Poobah face.

You're right: Thomas never changes his expression during play. He smiles when he wins, but by then it's too late! He does play it cautious though, and will probably fold early if he doesn't have a good hand.

Esme is similar: she'll probably drop out if the bets run high and she doesn't have a good hand. If she's betting aggressively and smiling though, watch out.

Sosi and Aeolus go the other way: they're liable to throw money down the drain on bad hands. This sometimes works to scare off the more cautious players (and Sean can take advantage of that too).

I wonder if putting the itching powder on Sosi's bed makes him play worse at the Midterm Poobah game.