Torah Temimah cites Shevuos 32a.
Until they hear a demand to testify directly from the mouth of the plaintiff.
Mentioning the case of the plaintiff pursuing them was necessary for Shmuel, as otherwise it could enter your mind to say: Since the plaintiff is pursuing them, it is like the case of one who says to them directly to testify. Therefore Shmuel teaches us that although the intent of the plaintiff is for them to testify, the witnesses are liable only if he tells them so explicitly.