One thing you must decide is why is this torture taking place? Is the tormentor some unbalanced person who loves inflicting pain? Is he a seasoned professional who must extract the truth from the victim and then go home to a more or less normal life?
Myself, I chose the second as they can be more devastating than some unhinged person who revels in the suffering of others. I made mine a pure professional - no malice towards his specimens and nothing but fondness for some afterwards - once he knows them heart and soul.
By that time, he has broken their spirit, many bones, shattering who they once were and making them but a shadow of themselves unable to recover. A broken will might recover, broken body can heal, agony fades - but when all are combined there is no recovery and he will grant mercy to those he has come to like.
I have a scene where my MC is captured and taken to a torturer. The person who takes him to this man (her father in-law) only listened to parts of the stories, tuning out the end where he would mention killing those specimens he had come to like.
I focus on my character’s attempt to deal with a situation over which he has no control and the certainty that this is where he will die.
My torturer examines his victim carefully, almost tenderly. It is his belief that the relationship between him and those he questions is intense and intimate. As wetcircuit says, at the end there are no secrets.
I briefly describe the mechanism and gloss over the array of instruments. I mention a few and that this man seems to blend the traditional with the modern.
My MC hopes to die quickly without divulging much, but doubts that will happen as this torturer knows exactly what the human body can survive. My MC knows this man, his reputation and what his brief future holds. He is a fly in the spider’s web with as much chance of escape. Freedom becomes death - after everything has been extracted from him.
I have the torturer rather suave and matter of fact, recognizing in this specimen a person with considerable training who might be a waste to kill.
The purpose of pain is to create fear, fear of greater pain. Eventually, the character will break unless someone intervenes.
Exhaustion will set in and the dance is over - no secrets, no lies and nothing left to hide.
Get inside both the questioned and the questioner. I found my torturer becoming rather chilling as he looked at my character as a piece of flesh to be reduced, not a person at all.
Manipulation can be a large part of it - the contrast between a gentle touch which the victim might well know is from page 50 of the book, and the agony to come can unbalance a person and set the torturer up for success.
Your torturer has all the time in the world, the victim does not and might not know this. Perhaps he thinks he can negotiate his way out - cooperate and be released. The torturer is in control at all times.
Perhaps your character holds on to hope and in his naivety believes this is something that he can both survive and transcend.
Is the torturer trying to confirm information? Is he doing it for fun? If there is a purpose behind it, that can make it more realistic and more chilling.