A personal essay is like a great conversation: an initial subject sparks an animated dash (maybe a measured stroll) that winds up and down among arguments, passionate defenses, mea culpa's, earnestness, humor, wryness, and unexpected digressions, the last of which make discussions between or among friends so interesting, and fun, and valuable. A conversation can go on for hours, or it can peter out in a few minutes. The shape stretches in many directions (sometimes at once) and when the conversation starts, the possibilities of direction and subject are infinite.
The danger comes when a conversation loses focus, when the center stops holding. That's when you get might get bored, restless, when sotto voce side conversations start, when the initial urgent subject dissolves into shallowness. What were we talking about again?
A familiar essay's center has to hold, no matter how far its limbs stretch, no matter how many issues it explores, no matter how many voices might enter, no matter how many digressions threaten its ballast. I've thinking about this lately about Montaigne's longer works. And Albert Goldbarth's many essays (that are in my opinion, undervalued). Also: Richard Rodriguez's great essay "Late Victorians" from Days of Obligation—it winds, it wonders about its many pieces, it delays, it forgets its opening subject and then returns; like a great, engrossing conversation (which it is, between Rodriguez and himself and between Rodriguez and his friend César ) it never fails to hold interest, it never fails to hold its center. It's a conversation that I love overhearing again and again.