If plaintiff’s counsel’s task is to make sure the client/witness doesn’t alienate jurors with a purely “they done me wrong” victim mentality, defense’s is different.
“Don’t whine” might be better stated “Don’t defend,” which is mightily challenging for defendants on the stand, who generally believe they are unjustly accused. Yet the defendant who argues with opposing counsel, whose testimony is a litany of “Yes, buts” and who attempts to evade plaintiff’s counsel’s most basic questions, will not find favor with jurors.
Instead, explain to your defense witnesses that during cross, at best, they will only be able to give a qualified “yes” or “no” (as in “At that time, yes” or “In that situation, no”), and at all costs must not argue with opposing counsel (“That’s not how it was, I/they. . .”).
Reassure your witness by role-playing with them how direct will go, not just by telling them “Don’t worry, I’ll unscramble all that in direct.”
The “can do” attitude for defense witnesses comes through on direct, when the witness, if and as is appropriate, educates jurors to the witness’ role, their experience, their situation. An attitude of imparting information, of sharing an experience, will gain far more sympathy with jurors than witness belligerence.