I need a better way to describe a witness to an event. Here is the sentence that has stumped me:
Terrance had been witness to the whole event, but since he did not feel like taking over as storyteller, he simply sat back and corrected any falsehoods in Smith's tale.
The last part of the sentence, from "he sat back" to the end, seems clunky to me. Is there a word I can use to explain his position? He speaks up only to make sure Smith does not fabricate any part of the story. I thought of describing him as a "testament", "corrector", "truth speaker", or "objective eye", but I don't know if any of these are correct.