By ‘people’ I mean “the species of which your protagonist is a member”. This can be anything from humans to Rfflikians from Nar’kzdos. In the end, if your story revolves around these “people” (for want of a better word) they will need to have emotions and customs just as humans do. Not necessarily the same way of thinking, but your readers, as humans, must be able to relate to your characters on a human level. Otherwise it’s just a scientific paper about the Rfflikians from Nar’kzdos and your only readers will be anthropologists who specialise in extraterrestrial lifeforms and societies.
Doesn’t sound so thrilling, does it?
Your characters may not be human, but they must have human emotions. Nervousness, fear, joy, anger – they must be capable of experiencing all these and more. So when it comes to emotions, you don’t have a lot of leeway. However, when it comes to what arouses those emotions, well, let the party begin!
When it comes to culture you can do whatever you like. Women can be raised specifically to be human sacrifices. Men can have the third toe on each foot removed at their 90th full moon. Wild buffalo can be eaten on the third day of the third month of every third year and the horns carved into sex toys. The possibilities really are endless.
Again, if you are stuck for inspiration, look to Earth’s own cultures. Every tribe has its own customs, most of which have been in place and revered as sacred for hundreds if not thousands of years. Looking back through history you will find many examples of things which to us seem utterly barbaric, but which to the people in question were simply a way of life. Example, Sparta. Closer to home, what about the common current practice of allowing teenagers a year off between school and university? You could put anything in there…
Again, you don’t have to lift something wholesale from Earth’s past or its present. Change a single detail and make it extraordinary. For instance, there is a tribe in Africa whose young men must leap cattle as a proof of their manhood. But what if a young man (or young woman) had to ride a wild buffalo to prove their prowess? What if all the men (and/or women) of that village/tribe rode buffalo instead of horses and their coming-of-age test was to tame a wild one?
Tribes living in the rainforest know how to live in such a way that their existence is not detrimental to their habitat and it is worth bearing this in mind when thinking up customs and traditions for your people. In these situations, conflict is often as simple as introducing a foreign race that does not have the same respect for its surroundings (think humans vs. Na’vi in Avatar). Or white men vs. just about any native race on Earth.
You could also turn this idea completely on its head and introduce a high-tech but nature-loving race that ends up dominating the not-quite-as-high-tech disregarding-nature race. I actually did this in my first novel and yes, the latter race was us. It produced some interesting questions, especially regarding the rights of the people who were there first, even if they were sending their planet to hell in a handbasket.
However, be very careful when introducing different civilisations to each other, as suggested above. It is very easy to make whichever civilisation is your bad guy somewhat two-dimensional – the big bad high-technology ignorers of Nature vs. the good, low-tech working-with-their-habitat people (or whatever). There should be pros and cons to both sides as creating a believable civilisation depends on all the various aspects being shown.
Civilisations are like individual characters in that respect. They must be as fully developed as the individuals that comprise them. When you get it right, you end up with a scenario where your fans want to know absolutely everything about this civilisation, up to and including its language (Klingon, anyone?). They don’t have to be the good guys to earn a place in your readers’ hearts.