A standard rule in another setting

Charles M. Schwab said, "All successful employers are stalking men who will do the unusual, men who think, men who attract attention by performing more than is expected of them."

Successful bridge players stalk tricks, which occasionally requires doing the unusual - as in today's deal. South is in three no-trump. West leads a fourth-highest spade seven. What should happen after that?

This is the single most common bidding sequence. South assumes that North will cover the spade suit, and North assumes that South will coast home easily with a combined 28-30 points between them.