By 'formal' I meant having a well defined, consistent list of technical (and possibly non-technical) questions that you will go through with every candidate. Hopefully the questions probe both breadth (various parts of math, computer science, coding ability, and beyond), as well as depth (drilling into an area the candidate is an expert in).

As an interviewer, I couldn't care less what the candidate is wearing or what he looks like (so long as he has basic respect for other people).