First and foremost, there are certain things that should never be discussed during an interview...
...but that especially you, in a position of potential vulnerability as an applicant looking for work, have a right to categorically refuse to answer.
If the interviewer is insistent you answer questions about any of the following, it should be considered a major red flag and you should reconsider proceeding any further in the application process with the company:
Please note that at a purely legal level, these do change from country to country, but nobody should be asking any of these regardless.
During the interview process, as early as your first phone call with a recruiter, you are likely to receive questions about how much you should be payed, so it is essential for you to be prepared.