The idea of filling in, mentally, it's very real. For example in many of what are called "hand poses", it is considered to be a best practice to show at least part of it all five fingers, and not to hide the pinky finger or thumb. Why? Because the longer a person looks at a picture, the more one starts to see the very small details that give a sense of visual completeness to the person.
When we are in the presence of a person, often times their personality or the stress of the shooting situation causes us to mentally not see everything, but when we see the final pictures we are confronted with every single error,omission, or shortcoming. This is where the term incomplete pose versus complete pose originated from. For example let's say I do a hand pose with the woman's hand positioned under her chin. Many people will do this pose incorrectly, and will have her rest the entire weight of her head on a fist, with the back of her hand facing the camera lens, when in reality the way to do it has been decided upon decades ago, it is a very feminine and dainty positioning of the hand with all five fingers visible to the lens,and which has been covered in many photographic instruction manuals, most of which are 50 or 60 years old now.
A lot of people look at old photos and laugh at the clothes or hairstyles, but if you look at old photographs of beauty queens or actors and actresses photographed in the 40s and 50s,they often look amazingly fit and trim, and attractive, because the majority of people who made the photographs back then were following sound posing fundamentals, and guidelines that have been worked out over literally millennia. Look at David--the statue David. He is posed expertly ! I have seen pictures of Marilyn Monroe and Scarlett Johansson, and both look great, because they were photographed by people who knew how to pose, and light, and how to use the camera well.
I have spent the last hour of my evening dictating to my phone a few of what I consider to be the real secrets to photographing people, to make them look good. I think that if you search out the right instructional material that your journey Will be easy, and productive and will stand you in good stead for the rest of your life. You will advance quickly once you learn a few fundamentals, none of which you are likely to find in instructional materials that were produced during the digital era.
If you wish to watch YouTube videos, I recommend the
Adorama TV series hosted by Mark Wallace, and the very good videos that B& H Photo has on their site, and the excellent studio lighting tutorials that Dean Collins did back in the 1990s, and the excellent lighting and studio photography videos that one can find on the Sekonic website.