His rule #1: is “Don’t Be Like Them” and his rule #2 is “Be Like Them”, but I’ll paraphrase his overall point this way: Speak their language fluently, but speak your own just as well, and never get the two confused. As Joss points out, this is good general life advice, but it’s also applies to both writing itself and a writing career.
In a writing career: Producers demands that you speak their language like a native, but they also demand that you have something of your own to say. Most film schools teach you to do only one or the other, please Hollywood or please yourself, but that’s useless because every writer has to be able to do both. Producers don’t want you to shatter their mold and they don’t want you to stamp-out soulless duplicates of what they already have, either. They want you to subtly twist their mold into a new shape. They want you to use their idiom to tell your story.
After screenwriters realize how brutally competitive this business is, the temptation is to get bent over so far backwards that you simply ask what they want and try to give it to them, but this never succeeds. No one’s going to tell you to be original and bold, because nobody but you is invested in your success, but they go crazy for it if you do it right.