He was positioned in the Dungeon's, having selected this over some other choice. There was no special reason that had led him here, simply the desire to keep Sirius away.
Ever since the incident in one of the old classrooms, he had felt nervous, even border lining on sick, when his mate just went past this place, as if the lure of what had been done would suddenly resurface.
It was silly, however he could not just dismiss it and so he found himself in the dungeons, where James had needed him to be for this spell to be a success.
Just to cast his part, Peter had practiced for days until he had gotten the knack of it, which let him feel the slightest reassurance as he raised his wand, the stone warming in his pocket, signalling the start of the prank.
Murmuring the spell underneath his breath, Peter cast the spell.