Before interviewing with Deloitte you will want to know all the essential facts about the firm. Headquarters, rankings, firm revenue, service lines, industries, etc., almost all of these can be found on Deloitte's website or Google. It is key that you have determined what service line you want to pursue before interviewing. Even it you are unsure, stick with one service and go with it because you can eventually switch service lines if needed. Also, you will want to have knowledge about the day-to-day activities for whatever particular service line you are interviewing for. If you happen to know about their regional industry (i.e. which clients Deloitte has), that is great knowledge to have. Of course practice tons of behavioral questions, opinion questions (i.e. what would you do if), and even "dumb questions" such as "if you had a super power what would it be?"
Interview at deloitte was pretty straight forward without any technical questions. A good amount of confidence is required as you will be interviewed by top managing people. Be ready to tell about your previous work experience if any and how it makes you eligible for public accountancy. You should have a solid answer about why you chose public accountancy as your career. If you are interviewed by big four they will appreciate a lot if you are clear about your long term goals.