In the movies and I think in real life, people are asked to swear or affirm that the testimony they give in court will be the truth:
"Do you solemnly swear to tell the truth, the whole truth and nothing but the truth, so help you God?"
If you are a Quaker and cannot swear oaths, you can substitute the verb "affirm" and you can invoke other important authorities, forces or conscience-binding things such as the head of a tiger or bear in some parts of the world. More detailed and interesting information can be found here on the Court Reporters Association website.
My friends have made clear to me that we mortals cannot tell what is "good" from what is "bad" without infinite knowledge. If my son breaks his leg in a fall from a horse, that could be bad but not if the broken leg keeps him from being swept up in a drive to forcibly draft young men in a bloody and murderous war. It is hard to know what leads to what so it is hard to know the full relevant story.
A problem with any story or account is that it begins after important events and ends before others. It is actually a problem of judgment as to what is relevant, what should be included in a story, account or narrative and what should be omitted. If you read my history of the US, you can always criticize it for beginning after the solar system was formed or ending before the important events that happened this morning.
I don't plan of smart-mouthing the attorney or the judge but I am confident that at some level, telling the whole truth is beyond my capacity.
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