Professional models know how to produce such expression at photographer command, normal people don't.
Still good photographer are able to get it from famous actors/politicians/sportsmen.
I've been asked to do some portraits for a friend portfolio, what should I ask for? Of course I don't want to ruin our friendship.
This question is related to How can I get my portrait subjects to look natural and drop the cheesy smile?
but it's also different because I'm talking about formal portraits, so I don't want a "natural" expression but a meaningful one.
This is also true for my own self-portraits...