The Principle of the Thing

Here's a sentence from Pajiba's otherwise mostly enjoyable review of "Indiana Jones and the Kingdom of the Crystal Skull":

I can’t believe this is the script that convinced the principles to make another film.

Is it really that hard to distinguish "principal" from "principle"? Lots of people seem to mix them up.

Here are a couple of easy rules to remember:

1) If it's a person, it's a principal.
2) If it's a rule or a law, it's a principle.

Generally speaking, if the word carries a connotation of "first" or "main", then "principal" is the correct choice, whether it acts as a noun (the principal, or main participant, in a duel) or an adjective (the principal characters in a movie). The most important person in the school: the principal. The main part of the money you owe, as opposed to the interest: the principal. (There are other meanings, too, but they're a little more specialized: one of the stops on an organ, for instance. You just have to know those.)

Both words, by the way, are derived from Latin "principalis", "first, foremost", and from this word "prince" is also descended, as well as "prime".