Usage patterns
〈人〉が証人になるfor someone to become a witness
He became a witness to the incident.
証人として出廷するto appear in court as a witness
She was scheduled to appear in court as a witness.
証人を尋問するto question/cross-examine a witness
The prosecutor questioned the witness.
証人を呼ぶto call a witness
The lawyer called a new witness.