Harry doesn't think that he's the most trusting person in the world. (With good reason, frankly.) He does think he's a pretty good judge of character. (With the exception of Quirrell in his first year but he was eleven then and he didn't know what a red herring was. He's grown since then.)
He doesn't understand why his best friends – who have previously trusted him enough to follow him to, if not hell, then certainly the animals that live there – are being quite so obstinate about his godfather.
Ron's managing to be subtler about his disapproval than Hermione – or possibly he's trying to come to terms with the whole Scabbers situation and that's why he's being so quiet. Hermione however, is leaning extremely close to him and whispering loudly about everything; she's focussing a lot on Harry's habit of jumping in head first (which isn't what this is), and on the statistics of health of released prisoners (why does she know this stuff), and dementors (Harry hates dementors. He doesn't want to think about them.)
He hasn't been able to catch every word of the mock interrogation going on in the main room, but enough of it that Hermione's litany isn't going to break through to him any time soon. Black talked about his parent's with such love. (He might have some of that love for Harry too.)
Only hours earlier, Harry had been thinking that Sirius Black deserved to be back amongst the death eaters and Black had had all of that inside him the whole time.
When the door opens and Charlie and Black step out, all of the eavesdroppers scurry away as quickly as possible and try to look like casual. Harry doesn't think it works – at least not for him, he knows, considering he's drawn to stare at Black like a magnet, even with his head bowed and Charlie pulling him quickly away from them all. Tonks and Lupin hurry after them, while Harry is stopped from doing the same by his friends.