What do the Sentient look like when they possess human bodies? Do they manifest tribal markings like tattoos to identify the different clans? How do they move? Where ever they are they will manifest a look and design to be able to live and fit in with that environment.
The look of the aliens, how they inhabit human form is the critical defining issue. It’s not just about FX make-up or FX. It is the body movement and acting. The series HUMAN demonstrates how that can make such a difference.