Take time to know your sitter
Over the years that I have been painting portraits, I have realised that the better I get to know the person I am painting, the more interesting the finished work.
It really is true that a face reflects the person behind it. So when undertaking a commission, I always take the time to chat and ask the sitter about their life and interests. Of course while they are talking I carefully watch their facial movements, their expressions and in particular how they make eye contact with me.
Hopefully they also get to know and trust me and to relax. Only when I am beginning to know the sitter will I start to think about and discuss the composition with them.
The creation of a portrait is a shared journey for both artist and sitter and in my experience, the better they know each other the more successful the end result.