Be prepared

The first three rules of doing well are:

i. Know your subject.
ii. Know your subject.
iii. Know your subject.

We were once looking for persons to sell sprinklers for agricultural use.

One of the interviewees was very nervous; he stuttered and mumbled incoherently when I asked him some general questions.

He was quite unimpressive and I was about to write him off.

Then, I asked him about sprinklers. He sat up erect, his voice became audible and his diction, very clear.

He gave wonderfully articulate answers to all my questions -- he was an expert in his subject and that knowledge gave him confidence in a way that even he was not aware of.