What is the proper etiquette in economy class?

A few weeks ago, Southwest Airlines ejected director Kevin Smith from a flight, saying that he exceeded the width of his seat. He countered by pointing out that he fit between the arm rests and was able to use a seatbelt without an extender. What if he didn’t fit, though, and asked the person next to him to keep the arm rest up? Is that person obligated to agree? What about a tall person sitting behind you who asks you not to recline? Would you oblige?