The simple answer is that the crucial element can be any element you want. HOWEVER, you bring up some other issues and they warrant a little follow up commentary.
If your Main Character is a FULLY archetypal main character, then the crucial element would be one of the MC’s eight elements (pursue, consider, actuality, knowledge, proven, effect, certainty, or proaction). Our Star Wars example, on the other hand, is a bit of a cheat. Though the eight principle characters align themselves at the Motivation level into a Dramatica Archetypal Character pattern (and therefore have the “appearance” of truly archetypal characters), the pattern does not hold up as you explore the three other levels of character: Methodology, Purpose, and Means of Evaluation.
Since Star Wars emphasis is decidedly NOT on character, the other levels are not nearly as well drawn as the character Motivations. This works reasonably well because the audience is given enough information to infer that the characters are archetypes. Encoding characters this way frees the author from having to illustrate every function of a character because the rest of the functions are implied. The author only needs to be explicit when a character represents a non-archetypal characteristic.