INTJs have Fi in the tertiary position, which makes it the function they long to be good at and learn from, but aren't very confident at. It is not a function they have great control over and can often be easily swayed by Fi-doms into changing the direction of their values, beliefs, judgments, much like INFJs can often be easily swayed by Ti-doms and their logical precision.
In my experience, IxTJs are very sensitive towards other people's moods and feelings, and extremely receptive towards the emotional atmosphere of a conversation. This is in contrast with INFJs, who are usually receptive towards people's motivations rather than their feelings.
The IxTJ's Fi shows up at odd spots, such as having sudden bursts of altruism, which may catch some people off-guard. Or expressing their love and attachment to the people that are close to them in their life, in stark expressions such as "I love you" or "I've been missing you". These displays may seem uncharacteristic, but they're an integral part of the IxTJ's personality and part of how they function.