With reference to Van Canna sensei's post on confidentiality, Student addressed the exceptions to the hearsay rule very well and the fact that in a court trial the inclusion or exclusion process often is dependent on how well counsel object and save their rights of appeal.
Because what is confidential varies in our different jurisdictions with rules in civil trials and rules in criminal trials, also having many exceptions or differences, one is wise to seek legal advise.
You must always remember that there is a fine line, in many cases, where you have the duty to protect yourself against self incrimination, or utter statements which would make you liable for slander or libel.
Depending upon circumstances, there may be a requirement that you give evidence or not to impede a criminal investigation. My advise is not to be a "good guy" or volunteer unless you seek legal counsel.
Allen M raises the issue of spousal immunity and its possibility of disappearance.
This will vary from state to state; apply in some states only to criminal cases, and may evaporate upon the disolution of marriage.