Legal interview? Is there a particular field that your agency does its work in (health, security, education, etc.)?
I've interned for two different federal gov't agencies, and both experiences have been very different. The way your interview runs will likely be influenced by the personality and focus of your agency. Therefore, it is hard to generalize.
I would treat the interview the same way you would treat any other interview: know the agency inside and out, know your resume from top to bottom and be prepared to answer questions about everything on it, try to tie your skills and qualifications to those that the agency is seeking out, and practice, practice, practice.
I'm assuming you don't since you posted your questions here, but if you know anyone who works for or has interviewed for this particular agency before, seek them out! They will be your most helpful informants. Additionally, your career services office might be able to give you some helpful pointers, especially if you are interviewing through them. There is also a Vault Guide to government jobs that you should be able to get a password to through your career services office. That could be somewhat helpful.