Legal Question in Civil Litigation in California

Only rebuttal witnesses need not be disclosed. Example: You expect the other party's witness to lie and say he never used steroids while he played for the Yankees. You have a witness who will testify he saw the other guy inject steroids in the Yankees' clubhouse after playing the game. This is a rebuttal witness. All other witnesses need to be disclosed (except expert witnesses which is a different subject). If you forgot to list a witness on your interrogatory responses, consider serving amended responses.

You need to disclose your witness list (minus rebuttal witnesses, as Mr. Stone notes) before trial. But the disclosure is not part of the discovery process, and its due date is usually after the discovery cutoff has passed.

However, if the other side *asks* you for this information during discovery, you usually must disclose it. Generally such questions take the form "Identify all persons whom you contend have knowledge of" particular allegations in your pleadings.

If you call a witness to testify about something but did not identify that witness when asked during discovery, the court is likely to forbid her testimony. Mr. Stone is right that, if you are getting close to trial and failed to identify your proposed witness in response to a proper discovery request, you should probably serve amended responses.