Basic interviews (for entry level positions) consist of about 15 questions and will last about 45 minutes. Interview will either take place in the main facility or at the site you are applying to work at. Applying on-line is most common. Some people apply at the specific site or at the HR office in the main building although it doesn't necessarily make a huge difference. If you apply in person, make sure you look nice and present yourself professionally as the manager will likely not necessarily speak with you but will ask others how you carried yourself, etc (and may even be watching from a distance).
If you're applying for Senior Management Position (including program director) you will likely interview with group of managers as well as the CEO.
More entry level positions typically only consist of one interview. This interview will be with an HR Rep and the Program Manager (along with possibly one staff member from the program).