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A very large number of timed choices which allow you to decide how the protag makes most of his conversation decisions. The vast majority of these choices have little individual impact on the outcome of the plot, but serve to help the game and its characters understand the character you're choosing to give to the protag.
More than two choices would be standard, and each would have "personality attributes" attached to them, such as "nice," "mean," "joker," "flirty," and so on and so forth. So a conversation option to pick on or tease a girl might be tagged with mean and flirty, adding points to those categories of your reputation. Of course, failing to make a choice in time would score you a lot of "quiet person" points.
Over time and enough of these choices to make it clear what kind of personality you're building for the protag, you start to get different interactions from the other characters based on their perception of your personality and their opinion of you. You could wind up locking yourself out of (or onto) a particular heroine's route simply by becoming a personality she either hates or likes. From a larger cast of characters, you make friends and open up social opportunities through these choices. In multiple playthroughs, you could wind up making completely different friends and enemies just by making different personality shaping choices.