How were they such powerful spokesmen? They simply believed in what they were selling, and their belief came through.
Here's Malden in the classic American Express commercial from 1985. The construction of this commercial rises to the level of Aesop's Fables. It is a simple tale with a moral, delivered with conviction.
Both Billy Mays and Karl Malden spoke with belief and authority.
Decades of persuasion research have pointed to source credibility as perhaps the key source of variance in message effectiveness.
There is a value in always telling the truth.