It is almost a cliche to talk about how important trust is in business. The only reason it is not fully in the realm of cliche is because there is too much truth to it to disregard.
Trust is not something that can be quantified. Like friendship, it exists on a spectrum that can change based on person, topic, time, or emotion. One day we may trust someone implicitly on any topic at all and then the next day decide we should not share basic everyday information. And the wild part of it all is how it is harder to re-earn trust than to establish it in the first place.
People form trust relationships 1-to-1 as well as within groups. There are unspoken rules that exist in any given trust relationship that are neither universal nor constant. Sometimes these relationships form due to proximity, shared job responsibilities, common interests, or even just by accident. You may not know that you had a trust relationship with someone else until something happens to require you to lean on it.
Another feature is that the relationship is not always equal. A casual trust to one person may have someone who is in desperate need for deep trust on the other. When the casual person disregards the boundaries of their trust relationship, it can have unexpected disproportionate implications on the other person. You can never know everything that someone else is going through. Someone who is normally strong and confident will have times of doubt and weakness. Someone who is normally laid back will have times of fear and anger.
Communication is critical to understanding the type of relationship you have with someone else. It is never wise to assume that you know how they will respond to any given situation. Just as you would hope that they would give you the benefit of the doubt if something goes unexpectedly the wrong way. Many movies would end at the ten-minute mark if the main characters would simply communicate and talk to each other.