If there is more than one or two of an alien race, they are always roughly the same size as humans.
Aliens usually speak english and have same colloquialisms.
All members of alien species wear the same outfits, including clothing, hairstyles, and jewelery. This makes them readily identifiable. Aliens who do not dress like aliens are hiding something.
This may, in fact, be a consequence of the fact that aliens all have single, monolithic cultures: one language, one religion, one outfit, per planet.
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