Look, it's more than important for you to be well prepared before any job interview happens. You'll need to, at the very least, have a cursory glance at the potential employers business and know the very specific details about the job opportunity you're interviewing for. You cannot enter an interview without knowing of this. Oh, and make sure you prepare questions in advance to ask - that way you're positioning yourself to learn more about this particular role.
Everything else, however, is far more difficult to prepare. In fact, I would say it is nearly impossible to prepare.
The fact remains that employers make decisions based on your fit culturally with the team already in place. It's a hard reality for many to swallow, but there is tremendous truth in this statement. The employer is studying your mannerisms, your tone, the way you keep yourself, and other variables before making a hiring decision. They do this not to discriminate; rather, they do this for the sake of stability within their current team.
You, too, owe it to yourself to study this organization - including the little nuances that may make it less than palatable for your career to endure. You need to be highly critical of this next employer, much in the same way they are critical of you and your candidacy. It's a matter of practicality. You have your own unique personality, and the employer must be able to see that personality be a reasonable fit within their organization. You, too, must judge whether or not the employer is a reasonable fit for your career.
Thus, the most important variable (arguably) in a job interview is not whether or not you are technically well suited for the role (although you ought not to submit an application for roles you are not well suited for), but rather it is your ability to 'mesh' with their team. Equal opportunity employers are not exempt from this, either; they want the best suitable candidate, and will never discriminate on the basis of any racial, religious, or other differences in each of us, but will be selective on the basis of your character and overall cultural fit with the environment they are looking to establish.
Thus, in a long winded way, I implore upon you interviewee's to prepare yourselves in a little more unconventional manner: prepare yourselves to be yourselves, albeit someone who is respectful of the workplace. You shouldn't act like someone else in an interview; you need to be yourself.
Allowing yourself to be authentic in an interview is crucial. That authenticity you display can make a tremendous difference in the interview; it gives employers a much better sense of what it is you bring to the table. And truth be told, most authentic personalities mesh very well with other personalities. You're doing yourself a favour, therefore, in being authentic.