In most cases, the interviewer will have a copy of your resume and letter, so you need to remember what you sent to this specific organization. In some cases, they will not have your resume and letter, so bring an extra copy just in case. You need to be able to talk about your past experiences without having to look continually at your resume. Review the materials so you can provide dates of employment and/or how long you worked for a specific agency. If you have to look at your resume, the interviewer will wonder if you really have the experience you claim.
If your resume says as an office manager you handled accounting, purchasing, and recruiting, then be able to provide examples of those activities with some short details. It is impossible to describe everything you did in each position without creating a 10-page resume, so have additional examples for each position, such as “assisted the organization with the implementation of a new Health Insurance Program.” Make sure you can provide details of your role and responsibilities.