Even if you receive a subpoena, you may not have to attend depending on whether the subpoena includes the required notices and witness fees, how far you would have to travel, and whether service was made correctly.
Without a subpoena, you are under no obligation.
No. You are not required to appear as a witness unless you receive a subpoena.
I am licensed in Nevada. The above information should be construed as legal recommendations only and not legal advice.