They will be looking for strong teamwork skills.
Chances are you will be asked to provide examples of when you performed very effectively as part of a team, or excelled yourself etc etc.
Dwell heavily on your flight planning and met experience. As an ATC assistant, I would expect that knowledge to be the one that shines through the brightest.
And make sure you have an in depth knowledge of the company that you wish to work for. It may sound pretty obvious, but some of the prospective staff members that I've helped to interview in the past turn up with NO knowledge of the company, or the job spec and that enevitably ends up in a very short interview !
Good luck !