What internal and external ?

In fiction, 'internal conflict' refers to a character's internal struggle. A character might struggle with an emotional problem such as fear of intimacy or abandonment, for example. ... External conflict, on the other hand, refers to the conflicts between a character and external forces.

Explanation:

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martians or other types of aliens represent a unique fear as opposed to zombies. vampires and other types of monsters.   in most sci-fi stories, aliens come from other worlds and have different or more advanced technology.   abductions and experimentation on human bodies that involve these technologies, or the use of these technologies against human beings and the use of unknown "powers", are a type of fear that's different from that of other types of monsters.