Look, I love the whole ‘James and Lily cast their Patroni in the middle of class and discover that they’re both a stag and a doe’ thing as much as the next person. But just imagine this adorable Patronus scene:

The DADA professor has invited all of her top students from each House, regardless of the year that they’re in, to a special class. It’s there that she shows them how to cast a Patronus.

Sirius is already smirking because he knows what James’ Patronus is and he has a fairly good idea of what Lily’s Patronus is going to be. He could care less about what his own Patronus is. He’s really only there to relentlessly tease his two friends when the time comes.

So his arms are crossed, he’s leaning against the wall, wondering what the house-elves are going to make for dinner tonight. He listens to everyone shout the spells. He sees wisps of silver light and a few feeble shapes that could almost resemble some sort of animal. 

And then the first fully-corporal Patronus is produced. Everyone’s gasping and awwing and clapping. Sirius can’t see what it is over the crowd.

But he hears James whisper, “Padfoot.”

And so Sirius confusedly makes his way through the crowd. He comes up next to James and screeches to a halt. He’s utterly speechless.

A large silver dog is running around the front of the classroom.

It finally returns to one Regulus Arcturus Black, who proudly lowers his wand before becoming very confused because his brother has tears in his eyes.