close values , matte painting and realism
one of the main ways that paintings can look "real" is the conservation of value and keeping value areas consistent. here's a great example. it's a matte painting by Peter Ellenshaw and you can see how narrow the value change in the distant background really is. by modeling within this narrow range the reality of the whole image is maintained. it does take great skill to do this and the smallest value change will be enough to indicate a difference.