Should I Call or Walk in?

I have been looking for a job for a while. My friend gave his two weeks notice at a place where he worked. I've applied to that place before, and I think I am qualified for the job. I know there is going to be a vacancy soon and that I should ask for an interview. Should I go to the place of employment to ask for an interview or should I call? I know I it is not guaranteed I get the job so which will make a better impression to the boss: walking in and asking, or calling and asking? which will more likely to get me hired?