Antonyms for the phrase "acted as a witness" would include terms such as "impartiality," "absence," "disregard," and "ignorance." Instead of being a witness, one could turn a blind eye, neglect the situation, or be uninformed. Where a witness is meant to provide testimony and corroborate events, antonyms would suggest disengagement and dismissal from the situation. The idea of antonyms allows for a greater understanding of language and context, and highlights the nuance of meaning that can be found within words and phrases.