Pre R1, Cassian is captured by bad guys your choice if humanoid or not, he is tortured or interrogated. K2 barges in eventually to rescue him but the bad guys are expecting him (maybe they were already suspecting cassian, watching him and learned the droid was with him ahead of time.) The bad guys manage to damage K2 pretty badly in front of Cassian (broken limbs etc) K2 is presumed dead/nonfunctioning and they leave his remains with Cassian who immediately tries to fix/reanimate him and he's successful but due to lack of tools he is unable to fix him 100% so maybe K2 is rendered motionless due to missing/destroyed limbs but is conscious, Cassian tells him to play dead when his captors return, so now K2 is witness to what Cassian goes through mission after mission when he is tortured/raped/interrogated and why he is always being sent to wait in the ship etc. Feel free to give it a happy ending where they are rescued or Cassian is able to put K2 back together.

+ K2 trying to keep Cassian's spirits up with jokes and Cassian trying to laugh through sobs/broken ribs etc.
++ Bad guys finding out K2 is conscious and using the threat of further destruction to make Cassian more compliant/obedient.