I remember someone (maybe Jay Sankey on his Restaurant DVD) saying that you should approach in interview with a mind to perform three effects. Any less may be too little - any more may be too much.
That rule has stuck with me and has served me OK - even when not in interview scenarios, three seems to be a nice number of effects to perform in one go.
I'd absolutely agree with Taffy - keep them simple... self working if possible (I know you won't like that, Tom!) but remember that the interviewer isn't there to be impressed with your magical skills but rather your ability to entertain his clientele.
Without dropping any names, a well known magician was at my gig on Sunday night and when chatting to him afterwards, he asked me how I had performed a particular force for a routine that hadn't gone particularly well. I told him that I hadn't forced anything, but rather had divined the details by a much more 'skilful' method. He thought for a long time and then said "Do you not think a force would have simpler, cleaner and easier for the audience to follow?".... and he was right!
Keep it simple and cut to the chase! Especially in a walkaround environment, you won't have a captive audience hanging on your every word desperate to know how you did things - they'll just want to be entertained.
Good luck with it
Sean