You're a detective, so you're going to scour crime scenes searching for clues. And when you question witnesses, you have to think less like a gamer and more like a sleuth. It's not easy. In fact, it's often a real challenge to judge the trustworthiness of a witness' statement. The line between "doubt" and "lie" is very narrow. Though L.A. Noire's hero, Cole Phelps, is regarded as one of the best case men ever, I still managed to falsely accuse dozens of suspects, doubt witnesses who were actually forthcoming, and use the wrong clues to try and catch killers in lies.
L.A. Noire is different. It's not like most video games developed by Rockstar. You don't play the outlaw running wild, free to kill, steal, and cause destruction. You're a cop. A good cop at that, determined to restore order to the violent streets of 1940s Los Angeles.