Snape nodded, though his mouth thinned to a grim line at the thought of trying to teach Sirius bloody Black anything. He'd just have to hope that some of the mellower mood the mutt had shown at the party would carry over to the lesson. But he sincerely doubted it.
Snape summoned a dozen of the slim phials he used for warbelts. "If you want to use combat potions, you'll have to practice throwing these. Fill them with water to get the weight right, and reparo them after each throw. Accuracy only comes through repetition."
Snape's smug look at James' enthusiasm over the Sectumsempra soured to a bitter glare at the next comment. "If you'd really rather I shagged, than invented that spell, then you're the one whose priorities need work, not I!"
Snape watched James cast the curse, and scowled, unsurprised by his less than impressive performance. "Cast like you mean it, boy!" he snarled. "If you expect to gut someone, you have to want it!"
He turned the scowl on James' Doppelganger, and it rippled like a mirage, its features blurring and shifting until they resembled Riddle's teenaged self.
"That should help you get into the proper frame of mind," Snape sneered, before giving a sudden, startling roar, "now CAST!"