Ron had been dreading his interview from the moment he realised he was going to have to give one. He didn't have anything useful to say. He was only here because he'd followed Harry, and he could barely look at Black let alone give information about his behaviour.
The good thing about the interview is that his parents can't follow him inside. (He isn't sure if that's right...it doesn't feel right. But the headmaster is in there, not to mention the minister of magic.)
The interview only proves him right: he has nothing of importance to impart. He spends every moment he isn't answering worrying about Harry.
Scabbers – Pettigrew – hasn't done a single interesting thing my entire life. The closest he's ever come to having a personality is biting Goyle two years ago. Perhaps that's a hint in itself; every other animal Ron's ever known – Errol, Hedwig, Crookshanks, Fang, Norbert, Trevor, Fluffy, and Hermes – they've all had something to mark them out as unique to their species (well Norbert and Fluffy might not, as he's never met another of their species, and never plans to.) Scabbers eats and sleeps and never does anything else. He's a useless, terrible pet.
(That's not true. Scabbers had had favourite foods and favourite places to sleep and ways of showing his appreciation or irritation at Ron. But Scabbers is a man and Ron almost wishes Crookshanks had managed to eat him before now so he could have died a rat.)
Ron stops himself from suggesting that they question Crookshanks if they want a good idea of Scabbers' suspicious behaviour, and falls silent.