Ha, then you haven't looked beyond the surface
Most INTJs appear like steel on the outside, but are warm and gooey on the inside. Teasing them is awesome, as is seeing them frustrated and unnerved at what you're doing. And, you know you're breaking through that steel surface when you're able to get them to stare at you and see the side of their mouth pulling up. It means they cannot help but be amused at you, and believe me, they are so trying not to be or at the very least not trying to show you
Yes they can be blunt, to the point, in a 'get out of the way, fool'-kinda way as they are usually on a mission. And seeing them on a mission is actually quite endearing and admirable at the same time, not to mention hot, as they ooze determination at that point. Now, if they're on a mission about silly details, then it is up to us, NFPs to smile at that and go tease them off that mission. Nothing like seeing an INTJ confused at your antics while they're trying to concentrate
It's risky, and you might get blown up at, but if you keep it lighthearted and just laugh at them, even if they blow up, and don't push them too far, most won't be able to resist your zaniness and crack up too. And voila...you have reached the gooeyness inside.. at least the first level
Believe me, they're awesome for when you need help getting shit done, as they don't beat about the bush. It truly is their strength. All they need in return is someone to give them a break from the mission on occasion