Last week I was attending a function at an old building. I had some time to kill so I decided to peruse an array of old class photos dating back to the late 19th Century. I love old photos, the faces of the men. Some look so serious, other look bewildered. I find myself wondering who they were, what they believed. As I was looking them over a friend and old colleague came by and struck up a conversation. He was switching assignments to begin supervising the crime lab and we soon began talking about how criminalists are different than other municipal employees. I drew his attention to the class photograph for mid-November 1963. "Check out who was President of that class" I said, pointing to the name. It was Jack Kennedy. Imagine the odds of that I thought (for our international readers that is essentially the name of the United States President assassinated 48 years ago today).
My old colleague replied "How do you notice things like that?" The answer was pretty simple, I made a career out of noticing the details. It's a skill that transcends the workplace too. Now, I'm not going to say that criminalists see everything or never make mistakes; but a good detective notices details. We notice things that others may not. I mention this to you for consideration in character development. Good CSIs and detectives don't just notice the small bloodstain on the car door handle. They notice when a friend is giving them a line of bull, or a policy doesn't make sense. We're trained to notice those things that run counter to the offered narrative.
I remember one time having dinner with a group of people. Most of which were friends of a friend, so I didn't know them very well. One guy got to the dinner late and his wife (who arrived earlier) wasn't too pleased. He claimed he had a flat tire but his hands and clothes were clean and he came straight to the table from the street (i.e. not stopping in the bathroom). A few months later I found out he was getting a divorce because of infidelity. I don't know if he was telling the truth about the tire but I couldn't help notice the inconsistencies.
So when you are writing a CSI or detective character keep in mind that their powers of observation are not limited to crime scenes. They will take note of little changes in their environment, their relationships, and are programmed to challenge those inconsistencies. It can actually be a little annoying at times. They may pick up on little white lies and in the process embarrass another character. One character that comes to mind is the television serial killer Dexter. He always seems to notice those little things that give him a special insight into the other characters. Ironically, the other characters don't suspect him of anything. I guess he embodies the devil is in the details mantra.
The point is that readers will expect your detective to pick up on certain details, especially if they are obvious. If they have to be tricked or deceived give them a good reason. Provide an alternate explanation for the evidence they can hang their hat on. Its a bit more work for the author but the payoff is well worth it. You also keep the reader in suspense and keep them happy all the way to the end.