The answer is always canon review. There will be some line or reference that sparks your imagination and you can bang out 1k easy.

Aside from that, finding another letter that inspires you is also good. Either that exact fandom, or a "similar" fandom. So if your fandom includes a long suffering straight-man and a OTT violent partner, go look at other police procedural letters. You will probably find something with a similar dynamic that you can use to inspire.