Should you stop in the middle of a demo?

I was near the end of delivering a demo to six representatives from a copier company. I had just demonstrated to them (several examples) of accessing features using a "single button push philosophy" that was one of the "hallmarks" of the system I was selling. This was in rather stark contrast to the popular feature access method at that time...which was pressing button after button of context sensitive menu choices. Just then...an individual walked in wearing jeans and sat in the corner. I thought about continuing the demo...but stopped and turned to the individual and asked who he was and what was his role in the acquisition of a new phone system? He said "I am John Smith and I sign the check"! I said..."Can you tell me what you are looking for in a new system"? He said...we distribute copiers and I can tell you the biggest problem is that users have to hit button after button of context sensitive menus and have great difficulty. So I am looking for single button access to features". Everyone laughed...including me...and I proceeded to show him why he didn't have to look further than the system now in front of him! Moral of the story...it pays to find out who enters the room in the middle of a presentation. Excellent Selling!