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In one of my first magazine-writing classes, my professor—a quiet type himself—pointed out that a surprising percentage of journalists are shy. His theory was that journalism is a sort of therapy. This might mean reaching that first scrum and becoming so enthralled by the rush that you forget yourself and have it at it. It might mean that the pressure you feel to achieve leads you to suck it up and do the work (which it doesn't sound like your school has been much help with). It might mean carving out a beat and gradually getting to know your sources and routines well enough that the shyness melts away. And it might mean finding jobs that are still journalistic, but not quite as socially demanding.
I'm not a naturally shy person when in close quarters with someone, but I have a profound fear of approaching strangers that played itself out in all sorts of strange ways when I was growing up. In part to avoid having to cold-call people under deadline, I switched out of television even though I'd found some success there and into a slightly more reflective field, magazines. I've done well by the route Cary suggests—being useful. I cultivated and developed my copy editing, software, and research skills. I tried to develop enough of an eye for visuals that I could complement the people whose job it is to create them. I basically threw myself into all the low-profile, unsexy things that no one else likes to do—and that also don't require me to spend my days well outside my comfort zone. I now have a high-profile job that carries a lot of responsibility, but requires me to interact mainly with a few dozen people who I work with every day.
The professor I mentioned at the beginning also emphasized the importance of taking the long view. Journalism is a craft—as you go along, you get better at every part of it, including interviewing and dealing with other people. And the better you get at it, the more confident you become when it's time to pick up the phone. You develop routines for sweet-talking people that arise out of your own experiences, as the letter writter who interviewed Johnny Rotten highlighted. The credit I've earned through muck-work has also afforded me the freedom to do interviews for stories I'm really engaged with, so that when I make calls it feels less like I'm probing someone who would prefer I didn't and more like I'm just really interested in what they have to say. As a result, I've gotten better at the interviews every time out, so on those rare occasions when I have to do the kind of interview I would normally dread, well, I dread it a lot less than I used to.
Journalism is inherently intrusive, and it often needs to be, but we don't all have to be Seymour Hersh (or write for celeb trash-mags). It's fine to confront your shyness on your own terms. The posters who say you've chosen the wrong field may well be right, but don't be too quick to agree.