Q: When is a person required to testify in beis din?
A: The Torah (Vayikra 5:1) states that if a witness withholds testimony he bears sin. Therefore, a person who knows testimony, who is qualified to testify, and whose testimony is of legal consequence, is required to testify if called upon by the litigants. According to some authorities, he is required to come and testify even if not called upon.
A single witness is also required to testify, since his testimony is of legal consequence in order to impose an oath. A relative, who is disqualified as a witness, is not required to testify, unless the litigants agree to accept him as a witness.
A witness who withheld testimony and thereby caused the litigant a loss does not bear an enforceable liability, but is liable b’dinei Shamayim. If the litigant grabbed payment for the loss from the witness, he must return it according to most authorities (C.M. 28:1; Pischei Teshuvah 28:1, 4, 6).