If you're good at smalltalk, keep talking... you know, about the summer, holidays, ask them if they've gone anywhere, or have anything planned, about their kids, etc.. Anything to get their mind off of their fear of sitting in front of the camera. Also, try to be positive in your posing suggestions. Rather than saying something isn't working, think of a way to change that aspect of the pose with positive suggestions.
My impression is that people go stiffer the longer the photographer is silent. I think it implies to them something is wrong and they'll assume it's something related to them, since they're in front of the camera.
I also try to keep the camera on a tripod for a couple reasons. It keeps framing relatively consistent from shot to shot but it also keeps you from totally hiding behind the camera. If you can keep some face to face, eye to eye contact, people will generally feel more relaxed.
Sometimes, for really stiff subjects, who I know will accept this after the fact, I'll make posing suggestions while shooting, then after a bit of this, sneak in a ridiculous order, such as jump up an down. If they do it, you can shoot it, but I always tell them I was just kidding... that I didn't say "Simon says...." It usually gets a good reaction. Might not be the right strategy for the CEO, though. 🙂
Something I've learned shooting weddings is it's pretty typical to try to get a bride or groom to relax by thinking about their partner. This could also work for your sitters. If you know they're married, or have partner, ask the person's name.. then if things are going downhill, sneak in "say 'John's sexy'." Another one that has worked, at weddings, is to order them to laugh hysterically. You can even lead them by example... 🙂
Since it's your office colleagues, perhaps reassure the more nervous ones that there will always be a chance to reshoot if they're unhappy with the photos. If this happens, try to have a mini critique with the person about what they didn't like about the photos to give you a better idea of how to proceed. My feeling is many people who hate sitting for portraits have had bad prior experiences.