I've noticed in many reviews of layouts that the very good Bachmann prepainted figures are frequently used. Most of the time these are used straight out of the box and, in this condition, to me, thay are flat and lifeless.
These are some figures I had lying around:
You can see that the figures lack shadows etc.
In addition to Bachmann, Woodland Scenics figures are excellent, Model Scene not so much. I've found that a very simple way to bring these to life is to apply a black wash (very thin acrylic paint). Dab the excess paint with a paper towel. The paint will be held in the crevices but flow off the smooth surfaces giving the impression of shadow lines and facial features.
Now this is very much a personal opinion thing. I think that the figures are improved. They will be at normal viewing distances on the layout and, like many things in model railways, we strive for the "impression" of realism.