A photographic portrait involves actively searching for and then instantly recognizing a SIGNIFICANT MOMENT.
The search is exciting.
Start with detached observation without the camera. Chat to the subject (or don’t) and generally get yourself and them ‘in tune’. Next have a think about light and shapes. Gut reactions count here. Use all the experience you have and also use your memory of other images or potential images.
Finally, LOOK. Get the camera to your eye and look for that significant moment like your life depends on it.