Also keep in mind that due to liability and registration issues, you will most likely be prohibited from any hands-on patient care. The "fly on the wall" approach is most likely all they will expect from you. It is great that you are working with the PD. I have "hosted" several applicants through shifts this year and through some observation, have a few "tips":
1) Be interested and active but don't get too much in the way.
2) Don't compare how the program you are visiting does things compared to where you have worked in the past - keep an open mind.
3) ASK THE NURSES HOW THEY LIKE THE RESIDENTS. This is a big one for me. An applicant who realizes the value of the ED nurses opinion is worth his/her weight in gold. Nurses have seen generations of ED residents come through and have great insight about where things are going.
4) Pay attention to the interactions between ED residents and consultants. If you feel up to it, curbside a consultant for their opinion of the ED.
5) There is more to learn from a visit of this type by simply observing. If the resident if stamping forms, drawing labs, starting IV's, and juggling other scut with minimal nursing support, there is no need to ask that resident if he does a lot of scut work. On the flipside, if there is down time (which happens rarely) get to know the resident on a personal level.
Finally, while it will be great to work with the PD, be sure to ask him/her if you can work with one of the interns and residents for an hour or so. ED residents most likely do so much behind the scenes that the PD may not give you a realistic view of what your job will be (but he will be pleased to work with you). You will be observed while you observe the program - be respectful, humble, and eager to learn (the holy trinity of the intern).
Just my 2 cents...
BKN would probably offer the best advice for the OP...