The most important part of portraiture (for head shots anyway) is giving clear guidance on how someone should move their skull so their face appears optimum, and they look confident, without smiling in a cheesy way. All people want to look hot and casually confident. And there is a way to do that!
IT IS ALL ABOUT THE CHIN. They need to stick their chin out way more than seems natural to avoid any kind of double chin, and then you need to get them to look head on at you and slowly lower their chin DOWN but NOT INWARDS. How much depends on the shape of their face. Ruth has a wider lower jaw, so she can dip down lower to slim it out, but if your subject has more of an oval face, they go less far down. You want the most flattering face angle so the eyes look large, but not stop before it seems that the subject is looking “up” at you.