Trust, at its core, is about how well we perceive  others of
of being free from pretense, deceit, or hypocrisy.  That is, are they sincere?

Our perception is filtered by a persons behaviours.  In short, what they say and do.  Sometimes knowledge we have about a situation can be used as a metric to determine someones trustworthyness.

Most often though our perception of sincerity is underpinned by feelings.  From a personal perspective there is great value in gaining the trust of others.

Earning Trust

Practicing behaviour that demonstrates sincerity should be simple.  It starts with the messages you send, whether these are based on  verbal or  non verbal communication, those must be 100% consistent.

Your honesty must be absolute.  One damaging action, leading others to question your sincerity, can take years to reverse.  The perceptions of others about you typically are developed quickly and changed slowly.

Sharing Everything

It may be an animal instinct originating from the birth of human kind; people sense when others are holding back.  A reticence to share everthing you know can impact the perception of your sincerity.

Most of us will all encounter knowledge from time to time which is not ours to share or can have negative results if we do (e.g., things bound by privacy laws or deemed to come under national security).

The key to handling these things is to be open about there existance and explaining why you can’t share them, in some cases it may be you have awareness but don’t know the answers.

Being open in this way can have a very powerfull effect.  Rather then letting others suspect you are holding back, you prove your sincerity with open disclosure to the extent possible, explaining what you cannot share and why (obviously not explicitly).