I'm doing a personal project taking portraits of friends, family and acquaintance. Most of them are not used to having their portrait taken and I don't intend to portray them as models either. I show them other portraits in the series beforehand so that they know what I want. I'm looking for nicely lit portraits without excessive makeup, just like they look in everyday life, but as flattering as I possibly can.
The problem is that almost everyone get very nervous days before and when they finally stand in front of the camera they've overdone their makeup, put too much thought into what they're wearing and they even look scared in the photos. When they've seen the first few shots they usually relax a lot, but they're still overdressed or have used too much makeup. Right now I often have to take their portrait a second time when they're confident I won't let them down and present them in an unflattering way if I want the natural look I'm looking for.
How should I approach taking planned portraits of people and make them feel comfortable and not feel the need to overprepare?