Memory is a tricky thing. In moments of chaos and trauma, witnesses tend to remember bits and pieces of whatever happened. Action, bits of dialogue and background sounds are like pieces of a puzzle without any picture to assemble the pieces against.
Writing a murder mystery is comparable to building one from bits and pieces of memory, gathered from all the by-standers who tried to remember what they saw in the few seconds or minutes of the crime-in-progress. A Writer has a lot of "what-ifs" to contend with. What if someone walks into someone else running away from the crime scene. 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 the twists and turns of eye-witness reports. It is so easy to taint an eye-witness's account of what he/she actually saw by having them talking to another potential eye-witness who perceived something quite different. This is why witnesses are normally kept separated at the crime scene and individual statements taken.
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 the readers see is not what they expect. Memory is such a nebulous thing and when it involves a heinous crime or murder, what the witness saw is not necessarily what he/she really saw.