I often ran into the same problem shooting corporate execs. Executives have very busy schedules and are used to paying a premium for the flexibility to accommodate their schedule. The way I handled this is to have them supply an assistant to stand-in during setup and staging (40m) and then have the subject arrive when I'm ready to go for 20m worth of shooting. I'd bill 2 hours for this, so I have a 30m buffer in case the subject's arrival is delayed.
In order to work under this kind of time pressure you need to have your lighting, posing and rapport techniques down to a science. Practice on family, friends, relatives, even mannequins (well, mannequins can be difficult to engage in conversation). Measure light-to-subject distances and tie strings knotted for the correct distances so you don't have to fumble with tape measures.
For an outdoor shoot I'd bring two strobes with a silver and a white shoot-through umbrella, or two speedlights with bounce cards, and hss-capable triggers, light stands, sandbags and a double-sided silver/white reflector, and then decide what is needed based on the location. I always use at least one flash to ensure I get some kind of highlight in the subject's eyes, even if the flash contributes nothing to the exposure.
Find out in advance whether the subject is married, has kids, his favorite sports teams and pet charities, and any topics that should be avoided. Then do a little research so you can carry on a conversation. Essential: make sure to ask if the subject normally wears glasses, and whether or not he/she will want to wear glasses in the photo, because this will impact your light setup and subject prep. If the subject wants to wear the glasses, ask if he can borrow a pair of frames without lenses installed from his optician for the shoot, as this will make things much easier.
When you arrive scout out the shoot location for good angles and then setup your gear. Use the assistant to take test shots and adjust. Be aware of any height differences between the assistant and the subject, because while it might be okay to have to raise lights and camera for a taller subject, it might be embarrassing to the subject if you have to lower them.
As soon as the subject arrives get him or her into position. If he is wearing glasses when he arrives but doesn't want them on in the photos, have him take them off immediately so the marks on the bridge of his nose have time to go away. Adjust wardrobe. Apply pancake if needed to reduce shine (have towels available to clean-up afterwards) and adjust hair. Have unscented hairspray to keep hair in place in the wind (make sure to use a hand to keep hairspray off the subject's face). Bring Have a hand mirror so the subject can confirm he looks good. If he wants to wear glasses during the shoot pay strict attention to reflections, shadows and distortions caused by the lenses (this is why using blank frames are best, if possible).
Most execs know how to turn-on the charm at a moments notice, and so might not need any time to warm up. But often those expressions will look forced or faked. So you need to engage him in conversation right from the start to get him comfortable and confident with you, and keep it up between poses.
During the actual shoot use your hands to show the subject how you want them to tilt, turn, lean, etc., instead of just verbal commands. Periodically review your images to make sure your exposure and compensation are good. Pay particular attention to glare, distortion and shadows caused by glasses. Don't show the images to your subject; you don't have time for that. Keep the conversation going and watch for fleeting expressions as you ask about children, sports, work. No matter what happens, do not allow yourself to become upset, flustered or impatient with the subject; keep smiling and confident. If the subject wanted to wear sunglasses during the shoot, ask if you can shoot the last 10 minutes without them.
At the end, thank the subject for his time, tell him it was a pleasure working with him (even if it wasn't), and explain who will be getting the proofs and when the will be delivered.
