Coaching facial expressions as a portrait photographer is a tricky task. In day-to-day life you don't consciously manipulate your face muscles the same way you do, say, your legs or fingers (unless you're a mime or a politician). Your outward expressions are the visible reactions to what is going on in your brain. When you try to fake an expression it usually results in a terrible portrait.
Prof Paul Eckman (University of California), is the godfather of our understanding of expressions. In the 1960's he built on the work conducted by Guillaume-Benjamin-Amand Duchenne (shown above performing facial electrostimulus) a century earlier and determined that the difference between a real smile and a fake smile is rooted in the signal generation in the brain; that is, totally different parts of the brain are responsible for creating real and fake smiles.
It all comes down to muscle control. You can consciously trigger the zygomaticus major, which creates a "fake" smile. But it's really tricky to tense your orbicularis oculi, which automatically contract during a "real" smile! Famed portrait photographer Peter Hurley figured this out after photographing faces for decades and teaches "squinching" to look cool when you're getting your portrait taken (Watch it! Please!!). Basically, squinching all comes down to--you guessed it--flexing the orbicularis oculi!
And then there's another little muscle problem... According to body language expert Vanessa Van Edwards, the zygomaticus minor is the critical muscle in a "real" smile. While it's easy to consciously flex the zygomatic major, and you can work your orbicularis oculi if you really try, the zygomaticus minor isn't as responsive to conscious control. In fact, only 1 in 10 people is capable of moving it at will!
Now that you're a real smile expert, do you think you can tell who's smiling for real, and who's a phony? Go ahead, take this test that the BBC put together. (I only got 17 out of 20. Must've been some mimes in the bunch!)
PS: I always heard that it takes more muscles to frown than to smile. ("Smile! It's less work!"). In fact, a genuine smile uses 12 muscles. A frown... only 11! If you want to know specifically which muscles are put to the task, read this rundown by plastic surgeon David H. Song, MD.