He was running blindly down the tunnel, not really paying attention to where he was going, just aware that he had to stop Sirius before he killed Snape. Sirius was notorious for not taking things seriously, and he was certain that he would do something stupid if he didn't get there in time.
He hit a dead end and cursed. He hadn't taken that false turn since the first time he'd come down here. Why oh why did he always screw up when it mattered most? If he made another mistake, Severus might be dea-
No. Don't think that. Just runrunrun…
He was almost there, just about… NO! He slammed into the locked cellar door that now blocked the passageway.
Stupid stupid stupid Sirius! He was going to. Bloody. KILL. Him when he got his hands around his skinny neck. Locking himself and Severus in there with a bloody WEREWOLF? It was all fine and dandy for Sirius, who could change into Padfoot at the drop of a hat, but, if Moony got out of control, what would happen to Snape?
He didn't have time to contemplate when he'd started worrying so much for Severus, or even when he'd suddenly started calling him by his real name, before he heard an ear-splitting scream of total terror.
NO!
He quickly changed into Prongs, a strong, regal, mighty stag. Without a second thought, he charged at the locked door. Thankfully, it gave quite a bit on the first pass, and he knew it'd only take one more good one before he'd be able to slip through. He didn't hesitate.
When there was a big enough gap he quickly changed into James before wriggling through the hole. He finally forced the last of his body into the Shrieking Shack, falling rather inelegantly on his face.
Another, louder, scream, filled with such complete agony that it nearly broke James' heart, had him scrambling madly for his feet- and his wand.
He screamed, "LUMOS!" And the sight before him froze his blood in terror.