The GB men are not your average person, they already know how to play the game. They also want to convince every JW of what they are saying and would know how to stage act.
At a guess, I would doubt that microexpressins are apparent when a person is giving a prepared speech, especially one that he has written himself or put into his own words.
You raise the point of noting their expression when they are being asked a question and this would be a better way of analysing their expressions. Isn't it funny that they have never allowed this type of question and answer from the platform with the congregation. It's not as if there isn't enough time to allow for this. In a learning situation this type of dialogue is an excellant way of learning and more often than not a question asked by a member of the audience is something most of the audience are usually interested in knowing the answer to. It also draws the whole group together in a friendly way and makes them feel as if they are working together. But like door to door witnessing the GB are not interested in the best way of doing things.