Little details matter, but there are so many little details that it is hard to tell which ones to focus on. You certainly can't process them all. So as cool as stories like Sherlock Holmes and TV shows like Psych and Lie To Me are, you can't just try to process little details to get at the underlying truth. You have to notice the right things.
Mokalus of Borg
PS - I guess that's where training comes in.
PPS - Most of us are observant enough for our lives.