No trial transcript of the Cadaver Synod exists. Nonetheless, it is reasonably clear what happened. Sitting on a throne, Stephen VII personally presided over the proceeding. Also present as co-judges were a number of Roman clergy who were there under compulsion and out of fear. The trial began when the disinterred corpse of Formosus was carried into the courtroom. On Stephen VII’s orders the putrescent corpse, which had been lying in its tomb for seven months, had been dressed in full pontifical vestments. The dead body was then propped up in a chair behind which stood a teenage deacon, quaking with fear, whose unenviable responsibility was to defend Formosus by speaking in his behalf. The presiding judge, Stephen VII, then read the three charges. Formosus was accused of (1) perjury, (2) coveting the papacy, and (3) violating church canons when he was elected pope.
The trial was completely dominated by Stephen VII, who overawed the assemblage with his frenzied tirades. While the frightened clergy silently watched in horror, Stephen VII screamed and raved, hurling insults at and mocking the rotting corpse.
There was a ghastly Trial once/Of a dead man by a live man, and both, Popes. (Metafilter; includes links to comic book version and Robert Browning version)