So, to make sure we've got it, Posting Character A is talking to Replying Character B when Interrupting Character C interrupts. If Replying Character B talks to both of those characters in the course of those threads and earns 10 comments out of it, it counts, even if those comments are split between A and C. (We're thinking of it kind of like a main thread and a subthread [the threadjack thread].)

Basically, if it makes ten comments in these kind of conjoined threads, then it counts!

Please feel free to count a thread + threadjack like you've described for activity.