I'll address the question in two parts - technical and personal skills. As for technical knowledge or skills, the managers and associates know that you are a clean slate, and they will not expect you to know much. However, I still suggest that you research or read up on the general audit process and how audit companies and the services they provide fit into the needs and demands of their clients. You don't need to take an audit course, a simple Google search and couple of industry readings should be fine. But understanding the bigger picture of the business, the client-auditor relationship, and the audit process will help you get the best out of the internship. Also as an intern, you will most likely be tasked with cash, AR, AP, employee compensation type of accounts. So, brush up on your accounting 101 and be comfortable with the accounts and transactions. Lastly, you should be very proficient in Excel. Although training week at GT will provide you with the basics, mastering those skills takes time. Therefore, I suggest practicing beforehand so you are comfortable working with large data sets and using formulas and Excel shortcuts on the keyboard. As for personal skills, you should always be ready to learn and be ready to take on any given task. Display professionalism at all times, especially on client-site.
In order to be successful as an audit intern you definitely need to be flexible in terms of your schedule and traveling. Be prepared to go to different client sites on different days and don't be afraid to come to work early or leave late. It is also important to have a positive attitude as the culture at GT is very friendly and open. Staying on task and completing quality work, while also finding time to socialize with coworkers, is important at Grant Thornton.