You can ONLY apply through their website. If you go to a local office, they will accept your resume and direct you to the website to apply, but your resume will not be passed on. If you know a current employee, they can recommend you, but special treatment will not be given. They will keep an eye out for your application, but you will be selected for an interview based on the same criteria as other applicants.
The first round is a phone interview with a recruiter. They will ask questions to clarify things on your resume or get more details about certain experience as well as why you are leaving your current job. They will also ask about willingness to travel and unusual/long hours. If you answer that you aren't flexible or don't want to work too many nights or weekends, you won't hear back. You will also be asked to complete a long online personality test to judge your compatibility with the position. The test is one used very frequently by hiring managers and unless you are rather dense, it's pretty easy to figure out which answers will behoove you.
After that round you will be brought in to an in person interview with the hiring manager and another supervisor. ACS uses the STAR interview system which is broken in to sections. The first section is background. Things like, "Tell us about your biggest failure or weakness." This is a good opportunity to talk about overcoming a difficulty (never say you have no weakness or failures) For the second section they will talk about you as you relate to the job. Be prepared to answer questions like, "Tell us about a time you had a conflict at work and how it was resolved" or "Describe a time you were overwhelmed at work. How did you handle this?" The third section is very job specific. Why you are interested, what do you hope to get from the job, how long you would like to stay, etc. They will give you an opportunity to ask questions. The last section is brief and mostly about what you want from your future. Where do you see yourself in five years, what is your dream job. This is when they will ask about salary expectations.
For certain positions (specifically, Relay positions) you will be asked a lot about your willingness to work long and unusual hours (nights, weekends, short notice) and the same rules apply as the phone interview - be flexible or you're out. They also talk a lot about "follow-through" and "accountability." Your answers to those questions are very important.