First impressions are very important, they set a standard for what people think of you. If the relationship continues, the first impression is layered over by more information, sensations, experiences together. The deeper a relationship goes, the more you know of someone, and are less likely to feel surprised at something they do. Or it could come as a complete shock when they do something that goes against the grain of what you think their personality is.
In the last month, friends who have known me for a couple of years or so have commented (with increasing irritation on my part) that they wouldn't have taken me for a football fan, or a sports fan of any kind. Amusedly they say in one form or another "I never knew you were into soccer/football/World Cup" . While I've laughed it off, it makes me think of what else would someone say "I didn't know you were into this" if they knew about me. And likewise, what would make me ask that question to someone I know now.