As Sun Tzu said in the Art of War, “If you know the enemy and know yourself, you need not fear the result of a hundred battles. If you know yourself but not the enemy, for every victory gained you will also suffer a defeat. If you know neither the enemy nor yourself, you will succumb in every battle.”
Knowing about the company you're applying to is not enough. You must also know yourself well in order to support the answers to any questions that the interviewer may ask you. Here is a worksheet that can help you record down what you need to know / or find out before you head into any interview.