The trick to doing any head shots or any other portraits is to get the person(s) to relax and to ease up a bit. I know, easier said than done, right!
Usually I know when I got the right shot for the client. It can be within the 1st 5 minutes, or it could be the very last shot of the session. Regardless of how many shots were taken, I still will take more then needed, just in case there is a break during the session where the client really lets their guard down, this is where I find I get the most “real” shots of them.
It’ not sneaking candids of them while they aren’t looking, it’s just getting to know your client either through small talk or chit-chat when they really let go and that’s where the real person comes out. You have to be able to read your clients and be able to use their nature and personality to navigate the session to bring out the the look your and your client are looking for. Just remember, the same approach will not work for every person, so that’s where you have to be able to read your clients and adapt your approach. Some will like talking, some humor, some will just open up after they get to know you.
It’s easy to say to your client, just pretend I’m not here, well that is hard to do when you have a big-ass camera in their face and maybe some soft-boxes and strobes going off close-by.
Look at some of the videos online of more famous photographers, like Peter Hurly, he jokes and talks to his clients all while giving them subtle directions during the shoot.
There is no magic trick or any single answer for this, I’m sure many others out there have a different approach, just have to go with what works for you and your client(s).