If you are like most people, you will be excited when a law firm or other legal employer extends you an offer for a job. Especially if it's your first offer, your only offer, or the offer you were hoping for the most, the inclination is to immediately accept. Even if you are not that excited about the opportunity, you might be inclined to accept immediately because you are afraid it might look bad to the employer if you hesitate. You might even be concerned that the offer will be revoked if you don't jump on it.
While you definitely want to show the same enthusiasm you did during the interviewing process, it is always prudent to take a bit of time to consider what you are about to do. After all, you probably would not commit to marriage after spending a few hours with someone. Yet, most lawyers commit to their law firms without much more thought-even though, in many cases, they will end up spending more time with their law firms than with their spouses.
At large law firms, you may be asked to go through one, two, or three sets of interviews. Even if you count up all the time spent on a screening interview and the callbacks, that really isn't a lot of time to get to know the firm or its people. While how the firm is doing financially, how it is structured, and how prominent its practice is are somewhat quantifiable, it may take more time for you to get a feel for the firm culture/personality and decide if it's the place for you. In many ways and for many people, the work environment is as important, or more important, than how good the firm's lawyers are at practicing law.