There are certain situations in which you, as a witness, have the right not to testify and you may refuse to answer certain questions.
Testimony against yourself or a family member
You have the right to refuse to testify against yourself or your family members. If you are forced to testify against yourself or your family, it may violate your right to a fair trial or the right to family life.
example If you have been involved in the commission of a crime and you are, or may be accused of it, you can refuse to testify if the answers to the questions would force you to incriminate yourself. Similarly, you can refuse to testify if your answers would incriminate a family member.
Confessional secrecy
A person authorized for pastoral care may refuse to testify on information, which was entrusted to him/her orally or in writing under the terms of secrecy as a pastoral person.
example A priest may refuse to provide information about a murder if he learned of the murder from the murderer during a religious confession.
Classified information and professional secrets
In addition to situation when you may refuse to testify, there are some kind of information that you can not be asked at all.
A witness may not be interrogated on circumstances relating to classified information, unless they were exempt from such obligation by the competent authority. The exemption may only be denied if State defence or national security is compromised or there is a risk of other equally serious damage. The reasons for the denial of the exemption should always be noted.
A witness may not be interrogated if their testimony could violate the law or the obligation of professional secrecy recognised or imposed by an international treaty, unless they were exempted from such obligation by the competent authority or those in whose interest such obligation lies.
example A lawyer must refuse to provide testimony on information he/she learned when providing legal services to his/her client. Such information is covered by professional secrecy duty. Only client can release lawyer of his confidentiality duty.
Moreover, a defence lawyer may not be interrogated as a witness on the facts which they learned during performance of the defence.
Private life
Your right to a private life, which is a human right, also has to be protected while giving your testimony. The official who questions you cannot ask you to reveal details about your private life which are not justified and necessary for the case. If that happens during the pre-trial investigation, you should file a complaint to the investigative judge. If that happens in the courtroom, you should ask the court to strike out questions which you think are violating the secrecy of your private life.