Study Vermeer. And Rembrandt. Their exposure times were stupid long, yet they made some pretty good portraits in natural light. August Sander, Henri Cartier-Bresson and Andre Kertesz also worked mainly with natural light.
Get people to look into the camera and imagine the viewer of the final photo. Then, if someone is nervous, I will NOT take the photo. Fake them out a few times. Wait until they crack up or settle down. Most people will put on a 'photo face' of some sort that they learned from their friends or family or Instagram, and most likely you want to get through this. Once they relax into the process, holding still for 1/8 second will be fine.