For the most part the cameras are pointed downward at the middle of the courtroom, where a couple of stone-faced people in matching robe-uniforms and a witch with a folder of parchments are leading a man who's presumably Sirius Black to a chair with chains on its armrests. He looks greatly the worse for wear: thin, pale, with a thousand-yard stare to rival any soldier's. He walks slowly to the chair with arms tucked close to his body, staring around like he's not exactly surprised at anything but is having a hard time believing it. The witch takes up station next to the chair, and the two officers retreat to the back of the room.
The witch in the judge's chair flicks her wand and the sound of a gong ripples through the courtroom. "This court will now come to order for the trial of Sirius Black on the charges of two counts of accessory to murder, one count of accessory to attempted murder, twelve counts of murder, and one count of flagrantly public magic. Amelia Bones, head of Magical Law Enforcement, presiding."
The prosecution goes first, in the form of another wizard with another folder of parchments that he clutches like a talisman. He goes over the story many people here already believe: Sirius Black was the Potters' secret-keeper, betrayed them to Voldemort, and when Pettigrew cornered him to confront him about it he blew up the street, presumably trying to get Pettigrew but only succeeding at killing twelve muggles. He apologizes for the lack of eyewitness testimony, but it's been twelve years and all the eyewitnesses would have had their memories wiped anyway.
Then the defense attorney takes a turn. She starts by calling in a mediwizard to testify that Black was in Azkaban for eleven years and only released to the hospital a couple days ago, and despite his very promising recovery is still getting the hang of holding a conversation, and there's no way he could be able to throw off veritaserum, at which point Black formally consents and requests to testify under the influence. The prosecutor has token objections, which Bones ignores, and soon Black is explaining the whole story in a magically compelled monotone. Apparently the Potters had worried about Black getting captured, and switched secret-keepers without telling even Dumbledore. When Pettigrew betrayed them, Black tracked him down and yelled at him incoherently until Pettigrew blew up the street and escaped as a rat. The prosecution asks why Pettigrew didn't surface for the following eleven years, and gets the admission that he probably feared Black's vengeance as much as Voldemort's, but wanting revenge isn't illegal and Black is released to the custody of St. Mungo's.
Pettigrew, of course, does not request veritaserum, but Black's testimony is admitted after some haggling. Lupin spends much of this section white-knuckled, but Pettigrew doesn't decide to bring up the lycanthropy issue. He looks like he would try bolting out of the room if it weren't for the guards and the fact that the chair came to life and grabbed his arms. He gets convicted of everything he did and sentenced to life in Azkaban, at which point he promptly loses consciousness and needs to be levitated out of the room.