Lip-service or heart-service?
The pasuk tells us that Esav was “tzayid befiv’. The common understanding of this is that Esav tricked Yitschok into thinking that he was really a righteous person and Chazal tell us that he would ask Yitschok halachic questions such as whether maser needs to be taken from straw, thereby misleading him.
However, Rav Dessler ZT”L quotes from the Arizal that these discussions were, in fact, genuine. Otherwise, there was no way Esav could have conned Yitschok. Nevertheless, there was a critical flaw in Esav’s mindset; all his righteousness was only “befiv”, it was all lip-service. In his heart he was the wicked Esav and there was no contradiction between his wicked heart and his “pure’ mouth.
This is what is meant in Chazal by Timtum Halev; referring to the situation a person reaches at which the Avairo causes a blockage between the heart and the mind; the sins of a person can actually prevent inspiration from penetrating a person’s heart.
Rav Yeruchem Zt”L would say that many people may have an inspiration or be bothered by their conscience. However, the only difference this might make to them is that it might diminish their pleasure from a sinful act without making any essential difference.
Similarly, there is an expression used among the Baalei Mussar that: Reshaim Mlaim Charotos - the wicked are full of regrets. The reason for this is that since the regret of a wicked person even if it is genuine is only external they never become absorbed by his heart in a way that will make him change his ways, and so the "regrets" just stay in one place.