Dear Davis, California:
In California, a party to litigation is always considered a witness or potential witness to a matter, even if they fail to list themselves on a pretrial statement. In fact, some types of pre-trial documents don't even require a party to list themselves as a witness -- although if the document you are referring to is a witness list, parties to litigation generally do list themselves.
Furthermore, under California's Code of Civil Procedure and its Rules of Court, a person who is in a courtroom can be called as a witness to a matter, even if they weren't given a Notice to Appear or a Trial Subpoena.
Even if the Plaintiff's side tried to exclude a Defendant from testifying (it would probably be done by a Motion in Limine to Exclude Testimony), a Judge that granted that could very likely have his or her decision overturned on appeal.
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