Show field based on dropdown value

Hi again!
As I am getting more and more into asprunner.net, the demands get higher and higher ;-)
What we want now:

I have a field status, which is populated through a lookup table. When a certain value for status is selected, say 5, I want another field, appointment_date to suddenly show up and become a required field. When status is something else the appointment_date should be hidden.
What would be the way to do this ?