Inventing customs, or leveraging Earth analogues, can help bring a species to life. Greetings, farewells, dining, and spiritual habits are all good subjects that will figure in our storytelling. Differences between this species and others can cause a misunderstanding, which has often been done in the Star Trek (italics) universe to the point of being a cliché, but it still affords us a chance to question human assumptions, which can in turn enlighten our audience. This is an opportunity to explore options other than those we’ve grown up with.
Actual customs can include shaking hands, holding doors for people, and ceremonies about birth, death, burial, weddings, and divorce. In a warlike story, soldiers might have customs regarding leaving their family, starting service, going into battle, mourning a comrade, and bringing a body home. Religion almost always has a number of customs associated with it.
This subject will be revisited in more depth in Cultures and Beyond, (The Art of World Building, #3).