I open up when I get to know people, then when I'm comfy I become a talking machine probably too often.
you answered your own question. You don't have time. If they want to live there, it's really their responsibility to make an effort to make a good impression. If they are too scared to try, then it will probably take them a long time to open up, if they ever doI don't like the idea of writing people off before I get to know them properly, but the issue with interviewing strangers and judging them is that you haven't got the time to get to know reserved people. I know some quiet people are very interesting once they do open up, but I don't have the time or money to give this special treatment to everyone that I interview. I'm not asking this solely from an "I'm desperate looking for a flatmate" point of view, I'm asking it in a general way too. How do you know if you're letting wonderful people slip by because you're in a hurry? It would be so great if there was a magic looking glass that could see into the mind and soul of individuals (though it would be frightening too, and could reveal too much information sometimes).
As I consider myself an introvert, I don't like the idea of writing people off just because they're introverted (like me) or reserved. Maybe I just forced myself to get better social skills to deal with interview situations. But just because someone isn't wonderful at social skills that doesn't mean they don't make great friends and mind mates. I don't like the idea of gravitating towards choosing people I can open up easily because I'm probably missing out on a lot of great stuff in quieter people too. *sigh*