There are three stages to the interview process: phone interview, skills assessment, and onsite interview.
The phone interview: For me, this came before the skills assessment, although some people do it after. This interview is very easy. I doubt they weed many people out based on their responses here. As long as you come prepared with questions to ask the interviewer, and aren't overly nervous, you'll do fine.
Skills assessment: This is probably the most important part of the interview process, and where most people get weeded out. This is also the most stressful part of the process, since they use a proctoring service that controls your computer remotely, and the tests take from 2 to 4 hours to complete. I spent 3 hours on mine.
Some people say that you don't need programing experience, but that depends on which set of tests you're given. There's a test that I believe everybody gets, where you're asked questions about a new programing language. For this test, it might be helpful to have taking a programming class, but you don't need it. However, there's another test that's much harder, that I got as part of my skills assessment, but some people did not. That's the test that has four questions that you're supposed to code answers to, and they get progressively harder. You're allowed to use pseudo code, but if you get this test, you absolutely will need to know the basics of how computer programs are structured (functions, variables, while loops, for loops, data types like strings, integers, and doubles, and what happens when you convert between them, singling out characters within a string and adding/deleting them, true/false bool variables, if statements, else if and else statements, and basic methods of data storage, i.e. arrays and how to use them).
For the harder test, everything I really needed to know, I'd learned in my introductory programming class, and I actually got side tracked on one of the problems attempting to do something more complicated that I hadn't learned as well, so to prepare for this test, I recommend sticking to the basics, or what you know best, and then making lots of comments on the test to explain your logic, especially if you're writing in pseudo code. Since the languages they use at Epic aren't that common anyway, I think what they really want is to see if you can think like a programer, and you don't have to get all the questions right to do that. Also, if you get this test, know that the people on this site who say the skills assessment was easy probably didn't get that test.
There are also some logic type tests that some people get. For the five minute test, and the easier computer language test, I think the key is to answer as fast as you can while still being careful, although clearly the five minute test is more intense. Other than the harder programing test, people are right when they say there's really no way to study for this.
Onsite interview: This is fun. It's a day-long process, but some of that time is devoted to lunch and touring the campus, and parts like that are mostly designed for you to interview them. There are some sections that can be stressful though, where you definitely are being evaluated. The case study is intense, and for me, they had me do it as an individual, not in a group like someone else on this site said. Also, there was computer testing. If you didn't get the harder computer test for the skills assessment, you may have to take it on site, depending on the position you're interviewing for. I was given the five minute test that some people get as part of their skills assessment, and then there was a longer logic and math test, still timed, but with enough time to properly finish and check my answers. There was also an HR interview, where the representative asked me some questions. This was more relaxed, like the phone interview, but I still could tell I was being evaluated.
For the onsite, don't even think about wearing anything formal, because nobody will. Nice jeans are good. Also, go through all the interview questions on this site. They're very helpful.