The teenaged male lead will begin the first day of the game by oversleeping, being woken up by his mother, and being reminded that he's slept in so late he missed meeting his girlfriend.
Any character who actually has a name is important in some way and must be sought out. However, if you are referred to as a part of a posessive noun (Crono's Mom) then you are superfluous.
"Silly Squall, bringing a sword to a gunfight..."
No matter what timeframe the game is set in - past, present, or future - the main hero and his antagonist will both use a sword for a weapon. These swords will be far more powerful than any gun and often capable of distance attacks.
Thousand Year Rule
The Ancient Evil returns to savage the land every thousand years on the dot, and the last time it showed up was just about 999.9875 years ago.
If You Meet The Buddha In A Random Encounter, Kill Him!
When you're out wandering around the world, you must kill everything you meet. People, animals, plants, insects, fire hydrants, small cottages, anything and everything is just plain out to get you. It may be because of your rampant kleptomania (see Garrett's Principle.)
Your characters must always keep both feet on the ground and will be unable to climb over low rock ledges, railings, chairs, cats, slightly differently-colored ground, or any other trivial objects which may happen to be in their way. Note that this condition will not prevent your characters from jumping from railroad car to railroad car later in the game.
These are actually just a select few... It's quite a long list and it'a a bit embarrassing how much we geeked out. But if you're interested in seeing the whole production... It's over here. ^^;