My father is ISTP. I'll guess I'll disclose something.
This is from an INFP perspective, and some of this might just be individual traits in him and not ISTP-behavior at general.
He loves his alone time. Fishing, hikeing, listening to music, that is what he prefers. If he has to go to a party or some other social obligation, he'll whine an awful lot about it, and drag his feet to the event, but when he is there he usually has a very good time. He can be incredibly lively, fun and exciting to be around, so to some it has been a mystery why he dislike being social when he is so good at it. I guess it's the inferior Fe, good at it but it cost alot of energy.
Moody, very moody. Alot of feelings. But not that sympathetic towards people: he love it when others make fools of themselves. Show negative feelings by being irritable (meaning not that whiny "poor me" moody, if he suffers he'll make sure other people suffers aswell

)
That "useful pain" attitude is in him too. It might hurt now, but it will be good again.
We have alot of fun when in good moods.
The biggest issue is the conflicts. Whenever I had done something bad, or was sad or whatever, meaning a situation where something was wrong, he became a bit aggressive or tough in his voice. That in turn made me shut my mouth, which made him more angry. Self-strenghtening tendencies in both of us, as it made me become more and more silent, and he becoming more and more angry.. . Some of my worst memories consist of him yelling at me for not wanting to tell him what was wrong when I cried.
He is not good at dealing with conflict-avoiding behavior. He is, however, excellent at neutralizing bullies. If someone act tough, he'll call their bluff.