I'm pretty sure that my ex-boyfriend is an ISTP. When would he open up and share his feelings? Umm, never. Ever.
There were two or three conversations over the course of our relationship where he did share something. It tended to be something that he had felt weeks or months ago, and he hadn't wanted to bring it up until now - perhaps because the feeling was over.
If anything ever bothered him, he absolutely refused to talk about it. If I asked, he would deny being bothered or tell me it had nothing to do with me, even when I was pretty sure it had. Then, he would say something about needing to go check on his boat or take care of some errand and disappear. I wouldn't see him until that night. When he came back, he would act as though nothing had happened.