Research. You need to know as much about the subject of your interview as you can in order to prepare a strong interview script. If the subject of your interview is an officer of an organization, call the organization and request a press kit. If the person you will interview is the subject of news reports or has written a book, get copies. If your subject is controversial, learn about the controversy. Read as much as you can about your subject and make notes about the areas you want to cover in your interview.
Start your interview with questions that identify your subject and place him in context. Follow these questions with those that address the issues you want to cover. If you anticipate the subject may be hostile or defensive, consider prefacing your questions with statements designed to set him at ease.
Number your questions and place them in the order you want to ask them. You might use capital letters for follow-up questions and lowercase letters for those you'll ask, depending on your subject's answer to your original question. It is difficult to cover all the bases in your initial script without having the answers, so being prepared with follow-up questions is always a good idea. Also be prepared for surprises and don't allow them to catch you off guard.
Make minor notes to yourself in the columns of your interview script to help remind you of the direction of the interview. This will help you stay on track. Many times during an interview, a subject may have his own agenda. You want to allow yourself room to improvise questions on the spot, depending upon your subject's direction; but you also want to stay on track and ensure your interview covers everything you wanted to cover. It might be helpful to write your questions under bold headings that indicate the general direction of specific sets of questions.