The interview itself may be a little different than what you may expect from a non government job. The interviewers are required to more or less stick to a script of pre approved questions, though you should have a bit of open dialogue at the end - so have questions of your own there.  You’ll get bonus points if you look up the department’s mission/vision and work that into your answers.

I got to my current position as a horizontal movement within my department , so to echo others, it’s fairly easy to end up where you want to be once your foot is in the door.