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One thing I see to make this come across as sad is to make it devastating to one of the more main characters. "He was my best friends, ex-wife's, brother in law from her third marriage..." isn't a...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/31757 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
One thing I see to make this come across as sad is to make it devastating to one of the more main characters. "He was my best friends, ex-wife's, brother in law from her third marriage..." isn't a very convincing connection, however "he was my best friend's brother, so we spent almost all our time together growing up as kids" can bring it home a little closer. Have a character concisely explain the connection and how it's affecting them. This sometimes can work with a non-main character being affected, too. Another way is to kill them off in a dramatic way, that brings distaste or surprise to the reader. "I had just met John, who seemed like a nice guy, but as he waved to me while crossing the road, a bus just runs him over as if he had never existed." Sometimes an anti-climatic way works, which sometimes needs a main character argument afterwards. "After hours of questioning, Lt. Smith just pulled the trigger, even though the prisoner hadn't done anything wrong, so I had to say something..." I'm sure there are other options out there that aren't so formulaic, so it'll be interesting to hear other answers.