Directors will just dismiss your image if you only have a blank expression.Keep the client engaged in conversation and directions so they won’t get bored.
Clients should have both open and closed mouth smiles, unless they absolutely insist on one or the other.
A good way to get a nice smile with the mouth closed is, after they laugh, you ask them to close the lips and to look at you with a friendly smile. Because they just laughed the smile will look more genuine.
Saying this, “looks like Your 99% happy”, will get the client to give a genuine smile.
How can one do curious expression?
Tell clients not to worry about minute details that no one will notice. It may give them a worried look instead of an approachable look.
It's better to book an hour shoot if a client is uncomfortable on camera. Half the session will be used up just trying to get them to feel comfortable on camera before we can even begin to focus on their expression. The rest of the shoot goes quicker because now they know what they're doing and we can get the other looks really quickly.
If clients have a specific issue, try your best to get them distracted from it so they will get some good expressions and genuine smiles.Or try having them tilt the head slightly to even the eyes, or hide a crooked tooth, etc.
Explain the difference between a real smile and a fake smile.
People think actors like getting headshots, but in reality, they don’t like the idea of having to ask for one moment in time. They prefer to act over a period time. So sometimes it takes them awhile to warm up to the camera so they should book at least an hour session with a few different looks.