Memory is a tricky thing. In moments of chaos and traumas, witnesses tend to remember only bits and pieces of whatever happened. Action, bits of dialogue. background sounds are like pieces of a giant puzzle--without that complete picture to assemble the pieces against.
Detectives try building that full picture, gathering from all the by-standers, what they saw in the few seconds or minutes of the crime-in-progress--a crime that happened right in front of them. Unlike a policeman who is trying to assemble all the facts, a writer has a lot of "what-ifs" to contend with. What if someone walks into someone else running away from the scene of the crime. And, what if the person running away is the witness's brother? Is this "runner" the person who committed the terrible crime of bludgeoning and robbing the store-keeper? Or, is he a "red-herring"--someone in the wrong place at the wrong time. What if this "runner" is actually chasing the real criminal whom no one remembers seeing? See--there's a number of "what-ifs" in what could have been a simple story of robbery and murder.
Writers have the imagination to drive Readers around the proverbial bend with all their twists and turns of eye-witness reports. It is so easy to taint an eye-witness's account of what he/she actually saw just by talking to another potential eye-witness who perceived something quite different. This is why witnesses are normally kept separated at the crime-scene and their statements taken separately.
So, how good is your observation skills? Are you doing the 360-degrees scan of the surrounding area when you're people-watching? Or, are you focusing on a specific individual. Whatever you're doing, tweek and twist the norm so that what Readers see is not what they expect. Memory is such a nebulous thing and when it involves a heinous crime or murder, perhaps what you thought you saw is not necessarily what you really saw.
In an August 30-2014 National Post article, "The Perils of Eyewitnesses: Overturned Conviction Shows Lack of Reliability," (firstname.lastname@example.org), the act of picking out someone from a police lineup can have its downside. Facial recognition based on a brief second of eye-contact of an innocent bystander at the crime scene can have devastating results. In this case, an innocent young woman was convicted of a crime based on one eye-witness report. Even a Writer may not have thought of this---only Real Life can show all its quirks and foibles.