It's a good idea to take advantage of the O*NET Web site (http://www.onetcenter.org), which provides job descriptions for thousands of positions — and it's free! In addition to the essential job duties identified by O*NET, you might also consider adding these items to your job descriptions:
1 - Being on time. (Yes, lawyers will argue that this is not an essential job function). If it's essential for an employee to show up at 8 a.m., due to the nature of your business, and flexible scheduling won't work for that position, then identify it as an essential job function.
2 - Mandatory overtime. If your business requires mandatory overtime, make sure that this is an essential job function. (Even though lawyers will argue that the essential work only has to be done within 40 hours per week).
3 - Rotation through jobs. Many companies do this for cross-training and other purposes. Again, if job rotation is important, put it in the job description. (This forestalls the argument that rotation is a punitive or retaliatory measure).
4 - A pleasant personality. For example, having a pleasant personality is an essential job function for customer service employees. (Again, lawyers will argue that being nice isn't an essential job function — simply getting the job done is).
Although it might sound ridiculous to add such requirements to your job descriptions, we've seen these issues arise in case after case, especially in the disability accommodation area.