Credibility – your job
Your promise must be honest, for a start. I can help you not to promise too much. And make any honest message plausible to the consumer.
Whether your message is a truthful one or not is ultimately up to you. That, I cannot much help you with (if it isn’t, I cannot help you at all). If you're not certain yourself, I can help you check.
I've had clients who believed in their own incredible message simply because they didn’t see an alternative. In some cases I've been able to point out why that message was implausible to almost everybody else, and to twist it until it was more in line with reality.
In other cases the message was perfectly true but so outrageously unusual that people just couldn’t believe it. In a case like that I can usually help the consumer to understand what they are hearing, thus increasing credibility by way of notching the third parameter, comprehensibility.