When I was in school my favorite teachers and mentors would tell me to never, ever ask your subject to smile.
When they told me this I was initially lost, after all who doesn't want a smiling portrait, were these people stuck in the 1800's?! As it turned out they knew something I didn't know.
That is they knew to ask themselves why they were taking the photograph. Rather than simply capturing a smile, they told a story with their photographs. Not every portrait requires a smile and your subjects certainly don't want to be asked to simply smile when they are paying you $300.00 or more for a Head Shot.
It's more marketable and far better practice to entice the emotion you need to tell the story your client has asked you to show, compellingly.