A staff auditor, also sometimes referred to as an internal auditor, is responsible for identifying fraud and mismanagement within a company, analyzing its financial processes to find points of inefficiency, and writing a report with recommendations on how to rectify any issues identified. Accomplishing this will require you to go through financial documents, interview staff, and may even include you going through communications like emails and letters. Staff auditors are almost always employees of the company they are auditing.
As with any auditor position, analytical skills are key. For this position, communication skills are also essential, as you will be trying to get as much delicate and sensitive information from the staff as possible.