Performance evaluations are a common part of business culture. Managers often do research ahead of these evaluations by asking an employee's co-workers about the employee's performance on the job. If your manager asks you to comment in this fashion, take it seriously, because it could make a difference in your co-worker's position with the company and overall career prospects.
Don't Be Hasty
When a manager asks you to comment on a co-worker's job performance, don't be hasty with your reply. Ideally, ask if you can have a day or at least a couple hours to think about it. This will give you the time you need to compose your thoughts and offer an objective, thorough response. Another way to avoid being hasty is to have your responses in mind ahead of time, before the manager asks.
Avoid Personal Bias
If your manager wants to know about a co-worker whom you happen to particularly like or dislike, don't let your personal feelings cloud your judgment. Offer an objective point of view, as if you didn't even know the person, and focus strictly on the employee's performance in the workplace. If that means saying that a friend has room to improve in a given area, or that someone you don't like is nevertheless a valuable asset to the company, do the right thing and say it.
Focus and Prioritize
The comment process isn't an opportunity for you to turn in a long essay. Be brief. Filter out all but the most important information, and prioritize what's left so that you say the most critical things first. Avoid going on and on, especially if you're commenting orally. If you're turning in a paper, try using the bullet point format.
Use professional, courteous language. More importantly, be tactful in how you say things. The wrong words can give your manager the wrong impression about what you mean. Aim for language that would look good as a quotation in the actual evaluation report, and remember that the way you choose to characterize your co-workers reflects on your own character.