First of all, I will apologize for my crappy rp as I have been learning some things that work, and some things that flat out don't - I was that taniel.
Now - to say that somebody spouting at the mouth and making threats is 'wrong' or 'unnatural' of somebody with low disc to me is a misnomer. While it may in fact be wrong or unnatural for some, many, or even most - that is making the assumption that
A - the person being tortured is in touch with themselves emotionally to the point of being able to acknowledge primary emotions
B - if the person is emotianlly disfunctional their low discipline means the very
instant torture begins they will just succumb and beg for mercy (a more emotionally functional response)rather than trying any way they thing they see to lash out in anger
To say it is easy for anybody to just play the impenetrable hero (guilty, damnit, sorry - but haven't for a while cause I learned) and that is why some people spit/cuss/fight with a low disc that should make them writhe in fear is making a very sweeping assumption that all characters will have the same basic phsycological tools for dealing with emotions/stimulations as a matter of discsipline.
I would love to see something disc based that forces tortured players to scream in pain or agony, flinch thier body parts away from torture devices, struggle wildly to free themselves(uselessly), or pass out from the sheer pain. These are all involuntary muscle responses at low mental disc.
I would absolutely hate to see it affect them showing emotion (ie offering information, begging for mercy, pleading for release, being super cooperative).
Purposeful torture is/has always been a physcological process:
Fight or flight are emotional instincts that each of possess, typically with one being more dominant, and both require discipline to overcome when it is appropriate to do so. The undisciplined response of somebody with a fight instinct, especially if they are emotionally dysfunctional (anger is a secondary emotion caused/related to emtional hurt/fear/loss/rejection

) would absolutely be to make stupid threats they have no hope of following through with, spitting, cussing, or any other completely irrational thing that pops into their angry little minds about how to lash out at who/what is hurting them. Of course, were the torture in question to continue for more than a few minutes before a person was cleansed/sacrificed then all might get the benifit of a character who can rp being worn down by the experience. A person with a dominant fight instinct isn't going to reason out to a point of wimpering/whining/cooperating until they are physically/emotionally too exhausted not too, regardless of disc. That would a prime example of how they are
failing to resist the torture - irrational words/actions that only worsen their situation.
Or do I misunderstand Disc and being mental discipline/ability to remain rational?
Maybe torture could have a visible effect on mana/stamina?
Anyway, rather than just assuming the present victim sucks at rp - which is always possible - you could try asking ooc if they are planning on being that way no matter what, maybe fill them in a bit on where you are taking the experience (if I think I'm dead at the end regardless, I'm trying to escape/hit back at any chance that shows itself and there goes your whip and release fun when I escape/you kill me), offer them a 'tip' about how boring/weak it is if they intend to stay that way, and then shorten or lengthen your rp with that char accordingly.
You can lead a horticulture, but you can't make her think.