Trust in a relationship begins with a mutual respect for one another. Complete honesty regardless of the pain it may cause is crucial in a trusting relationship. This applies to both personal and professional relationships. There are certain people I work with who I would choose to be 'pod partners' with over others. Those are the ones I can trust to jump on CLEER (Colorado Law Enforcement Emergency Radio) channel and notify a neighboring jurisdiction of an incident I may be working. When in the middle of a critical incident, it's important to trust a coworker to notify other agencies, make calls, get the dogs to respond, etc when you can't.
Personally, it's crucial to maintain honesty in a relationship. Regardless of how painful something may be, it's important to be honest. Betraying that will devastate a relationship and any trust that did exist will be gone. Open communication with one another is important. Some people feel it's necessary to 'sugar coat' things so not to hurt anyone's feelings. Personally, I find the trust in my relationship is stronger because when necessary I can be brutally honest.