My best friend in the universe is an ISTP. I wouldn't say 'shy' was a word that really comes to mind about him. Reserved perhaps, but not shy - shy is my sister, who's been known to go a long distance just to do a certain thing, and then seriously consider going back just because when she gets there, it isn't immediately apparent which door to go through and she won't ask the receptionist or whatever. THAT is shy.
My ISTP wouldn't have any trouble with asking strangers for the time/a light/directions etc. and when it's non-strangers at a dinner party, but people he's still not very familiar with, he's still fine to ask them to pass the corkscrew or any question he feels like asking (unlike my sister, who'd go without wine rather than ask for the corkscrew).
But on the other hand, he can sometimes be a bit overly self-effacing, needlessly so, when he talks. I think he thinks he's less bright than he is in reality (he is very bright), and sometimes assumes that whoever's in the room is bound to be cleverer than him, so I often see him venturing opinions or information, then sorta trying to dismiss his own words as if applying 'attack first' to himself, like he's sure everyone'll think he's stupid or wrong or whatever.
Particularly if a 'big word' is involved in articulating his ideas - he'll say the word perfectly, and I know that the only way to make him keep talking is if I show no reaction whatsoever to his use of this word. And whilst in normal circumstances I wouldn't even notice, it's hard not to notice or react when I can see that he's suddenly all nervous and looking at me like he's trying to see if I'm inwardly laughing at him for pronouncing it wrongly or something. And it's weird cos I've never ever corrected him once. He's rarely ever needed it and as often as not he can correct me. But I have to just not even blink and act like I haven't noticed he used a big word, if I want him to just carry on with what he was saying (which I do).
And he stops himself talking all the time, starts telling himself to shut up right when he's really starting to open up and pique people's fascination.
I dunno, it's like he has a self-destruct mechanism: he believes everyone thinks he's dumb, but then he creates a viscious circle by not letting himself say anything that would prove otherwise.
He's still a top dude though. I mean he really knows his stuff.