What is the protocol when you have an offer in hand from Company A but you're waiting on a second job offer from Company B that you think would be a better fit? Let's assume Company B's offer might not be forthcoming (if at all) for 3 weeks or more. Obviously, you can try to tell Company A that you're sifting through all your alternatives and need 2 weeks, but how long can one reasonably expect to string along Job A? I've seen people accept Company A (a bird in the hand...) and then quit in a month if Company B's offer comes through, yet that strikes me as quite unprofessional. What do you think?
Yes, it's absolutely unprofessional--and unwise--to accept a job only to quit soon after if you get a different offer. The first company will have turned their other candidates loose by that point and invested time and money in preparing for your arrival, so this will burn that bridge in a fiery explosion. You'll also damage your reputation in your industry; people talk and others (whom you may someday want to work for) will hear about it. So assume that once you accept an offer, you'll need to keep your word.
However, if you're in this situation, here's what you can do. First, tell Company A that you're extremely interested and you want some time to think it over. Any reasonable company will give you a few days or a week. Most companies won't want to give much more than a week, though, because they have other candidates they need to get back to. Asking for more than a week is also dangerously close to code for "I'm hoping to get an offer from somewhere else I'd rather work," and they'll begin questioning your interest level.
Next, contact Company B immediately. Explain that you have an offer from another company that you need to give an answer to within a week, but that Company B is your first choice. If Company B is very interested in you, they will expedite things. If I have a candidate I think I might want, I will move quickly at that point; I've even cut a vacation short in that situation in order not to lose the candidate.
However, do be prepared for Company B to tell you that (a) they can't move up the interview because of schedule conflicts or (b) they don't expect to be able to make an offer decision within a week. If that happens, then you have a hard decision on your hands. Are you willing to turn down the offer you have, without any guarantee that you'd get an offer from Company B in the future? It's a calculation only you can make. Alison Green is chief of staff for a medium-sized nonprofit where she oversees day-to-day management of the staff as well as hiring, firing, and staff development. She is working with the Management Center to coauthor a book on nonprofit management. Her writings have been published in the Washington Post, the New York Times, Maxim, and dozens of other newspapers. She blogs at Ask a Manager.