Part of trust building takes place during every interaction between two people. Trust is being built as a function of the time spent together, the intimacy of that time, and the positivity of it.
The typical level of intimacy can be measured by the answer to the question:
How much of what I think and mean was I successful in conveying?
Albert Mehrabian, in his 1971 book Silent Messages, claimed that 7% of that would be conveyed through words, 38% through the tone of voice, and 55% through body language.
However, there is a deeper level of intimacy than conveying what I think and mean, even with 100% accuracy. This one would answer this question:
How much of what I really believe did I convey to the other person?
What I believe is deeper than what I mean. Sometimes I don't even know myself, consciously.
You have much stronger conscious control over what you say, the words you use, than over your non-verbal communications (your tone of voice and body language).
Strong communicators, highly trustworthy, are those who can consciously convey to the other person what they believe. They translate what they believe into what they think and mean, and then are successful in conveying that through words, tone of voice, and body language.
Others will subconsciously convey what they really believe, and consciously convey only what they think and mean.
In his book, Albert Mehrabian also said:
"When our words contradict the silent messages contained within them, others mistrust what we say."
If what you believe and what you think and mean are the same, that's not a problem. But sometimes, as a defense mechanism, or as a way of manipulation, we mean, think, and consciously convey something different than what we really believe. That's when we are fake and disingenuous.
However, as Malcolm Gladwell so eloquently described in his book Blink--The power of thinking without thinking, we can subconsciously sense what you subconsciously conveyed, even if you didn't intend to. If what we consciously sense through your words, tone, and body language is different than what we subconsciously sense from you otherwise (maybe partly through your tone and body language), we lose trust in you.
We are pretty good at that.
So if you are conveying something different than what you really believe, just know that you are losing trust. Don't do it.
Trustworthiness is the most important quality for your success.