If you have trouble keeping secrets or delight in telling bits of information about others to anyone who will listen, you must learn to tame that tongue. Not only can revealing sensitive or secret information put your case at risk, it can put your safety at risk as well. When a client pays you, it's assumed that any information you discover is for her eyes only. In some states, a PI can be prosecuted for leaking client information. The likelihood of prosecution rises dramatically if the leaked information has been procured for an attorney or law enforcement agency. Even a casual remark to a friend can prove disastrous. Not only can you not control information once it's in another's possession, but you never know who may overhear.
If you spill the secrets of others to feel important, procure a confidante, gain notoriety, or obtain money, PI work is not for you. Secrets kept by investigators affect the lives of real people. If you have no understanding of this and no empathy for those whose secrets you hold, you're not investigator material.
Read the American Bar Association's Rule 1.6 of Client/Lawyer Relationship's guidelines regarding confidentiality. The main tenant of ABA regulations is that a client must give informed consent in order for the attorney to disclose information. An attorney is allowed to reveal client information only if she believes it necessary:
To prevent reasonably certain death or substantial bodily harm
To prevent the client from committing a crime or fraud that will result in substantial injury to the financial interests or property of another (meaning the client has used or is using attorney's services in furtherance of this crime)
To prevent, mitigate or rectify substantial injury to the financial interests or property of others that will result from the client's criminal act (meaning the client has used or is using attorney's services in furtherance of this crime)
To obtain legal advice concerning how the attorney should comply with these Rules
To establish a defense on behalf of the attorney; to establish a defense to a criminal charge against the attorney based on client conduct; or respond to allegations regarding the attorney's representation of the client
To comply with another law or court order
These are reasonable guidelines for investigators. In fact, several private investigative organizations have used ABA Rules as a basis for their own guidelines.