Defense witness who does not want to lie

You have to be truthful while testifying on the stand under oath. As a court witness, because the potential repercussions of your witholding of information prior to your in court testimony, your conduct may result not only in your impeachment as a witness in that criminal trial, If you made prior submissions and then testified otherwise. There are also potential charges of obstruction of justice if you withheld information about the crime you knew was relevant.
The rule of thumb in your scenario is to be honest and truthful before giving the in court sworn testimony to avoid potential complications.

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