I love this game and one of my favorite games, and underrated.
I will say this. You're solving a murder. The murder happens the same way no matter what your choices in character development are, and the killer is the same, the suspects are the same, the progression of the investigation is generally the same (there will be variations depending on) but however choices do indeed matter, for example, I went into a certain scene without a weapon, and multiple characters were killed off for good, and though out the remainder of the storyline my character kept commenting on their pain. The main plot kinda stays the same, but choices in the sense of how much knowledge you have going into a situation, or who lives or dies, or what you notice about certain things.
When I played 2nd playthrough with different Harry personality, Harry will end up being a different person. You'll get different background information, hear different feedback on your choices. The dialogue options are completely different and I've had to handle several scenarios than my first playthrough