on the stand
Testifying in court. "The stand" refers to the place where one sits when testifying. If that stool-pigeon gets up on the stand, our business will be destroyed. We've gotta shut him up, one way or another. I know you're nervous about going on the stand, but just tell the truth and everything will be all right.
on the (witness) stand
[of a witness] giving testimony in court, seated in place in view of the court. I was on the witness stand, answering questions, when the judge declared a recess.