Reading an extract from her forthcoming title What Happened on news network MSNBC on Wednesday, Mrs Clinton said she did not know how to react as the pair took to the stage for their second debate last October.
"Do you stay calm, keep smiling, and carry on as if he weren’t repeatedly invading your space? Or do you turn, look him in the eye and say loudly and clearly: ’Back up you creep, get away from me. I know you love to intimidate women, but you can’t intimidate me, so back up.’"
The debate took place just days after a 2005 recording was publicised, in which Mr Trump bragged of grabbing women by their genitals and getting away with it because of his status.
After an uproar, he apologised and dismissed the conversation as "locker-room talk".
In her forthcoming book, Mrs Clinton says this made her feel even more uncomfortable on the night.
"This is not OK, I thought," she writes. "It was the second presidential debate and Donald Trump was looming behind me. Two days before, the world heard him brag about groping women.
"Now we were on a small stage and no matter where I walked, he followed me closely, staring at me, making faces."
She says she chose to respond by gripping her microphone tightly, but she sometimes wonders if she should have told him to back off. "It certainly would have been better TV," she says.
"I kept my cool, aided by a lifetime of dealing with difficult men trying to throw me off [...] Maybe I have overlearned the lesson of staying calm, biting my tongue, digging my fingernails into a clenched fist, smiling all the while, determined to present a composed face to the world."