"I was never told that!" If the EAP confronts your employee about job performance problems that have not been presented, the employee will become angry and distrustful of you and the EAP process. The EAP can't play the role of supervisor and confront the employee about performance problems that he or she has not heard about from you first. Some supervisors think that the EAP will do a better job at the delicate task of confronting the employee about specific job performance problems that may provoke the employee's ire. Unfortunately, it doesn't work that way.
The EAP doesn't need to know everything. Similarly, do not provide information about behavior problems or performance issues that you do not want shared with the employee. Information you want to remain undisclosed is generally not necessary for the EAP to know. An interview with the employee is made more difficult, and the risk of this information being accidentally disclosed during the interview is high. This will spoil the relationship of trust between the employee and the EAP that is critical to an assessment.