Make a good final impression. If you ace your interview and you're offered the job on the spot (yes, it happens!), don't feel pressured to give an answer right away. It's okay to sleep on it. Some people worry that saying that they need time to consider an offer will signal a lack of conviction. But as an employer, when someone tells me he needs to think about an offer, it makes me think he has other options. If I'm really impressed with the candidate, I might even improve my proposal if I'm told that the uncertainty has something to do with a competing offer by another firm. If this is indeed the case, be sure to say so—and to express your excitement and appreciation. Promise to get back quickly with an answer. What I would not recommend, however, is asking for time to consult with your spouse. I'm sorry, but in my book that's a weak answer. Unless you're talking about a job that will require you to relocate and uproot your family, don't let it appear that you're putting the decision on someone else. If you just want some time to think about it, just say so. But never give the impression that you don't make your own decisions.
I was lucky when I graduated from college, enormously so. I had a clear idea of what I wanted to do and how to accomplish it. I also had an edge: there's no denying that my family name, first-class education, and top-tier contacts gave me a bit of a leg up—a "Trump" card, if you will—but I'm also a firm believer in making your own luck and making the most of your opportunities. Remember, that card will get you somewhere only if you play it to your advantage.
These days, I know a lot of newly minted college graduates and recently unemployed young professionals who feel compelled to go out on as many job interviews as possible, even for jobs outside their areas of expertise. (This can sometimes work, provided the jobs are within an area of interest—because if you don't quite know what you're doing and you don't quite love what you're doing, you won't get very far.) In any case, they're doing what they can to make their own luck or maybe to turn a string of misfortunes in a more positive direction. I applaud their initiative. After all, if you need a job, you're smart to go where they're hiring. However, if you're looking for a position that might not seem like an obvious fit, it's up to you to explain how you can match your skills to the job at hand. Don't expect the interviewer to make the connection or the extra effort.
Most of us never get to be on the receiving end of the kind of call I received from Anna Wintour back in college.
There, the tables were turned a little. Anna was asking me to consider a position that was a bit outside my wheelhouse, so I didn't have to sell myself or make her see how I might adapt to a new situation. She was coming to me because she already believed in my abilities. But that's not how it usually goes, and it's unrealistic to expect an unlikely job to come looking for you. It might happen from time to time, but you can't expect time to time to come around on your calendar when you really need it to.