jamayca5.. There is no interview at stage 1, you just have to do the various aptitude tests including the multiple choice test based on the material you can find on the internet regarding ATC. In stage 2 you have a HR interview and in Stage 3 you will have a combined HR/ATC interview.
Goodluckage
p.s. scroll to earlier posts in this thread theres some good prep info in there for the tests.