Swap out one of your characters functionally for another. See what transpires.
When you already have a first draft. Additionally, when other methods have failed, you're at your wits end, or you'd just like to ask yourself "what if."
"A chord that can be substituted for another while retaining its harmonic function," is a substitution chord according to the Harvard Concise Dictionary of Music and Musicians. In general, the idea is that the chord is not uncomplimentary to what surrounds it. It doesn't stand out. It fits, but it does change things contextually.
Unlike the Chord exercise previously, this exercise asks you to not simply change one of the notes within a chord -- a meaningful but overall subtle change -- or even add a new note to an existing chord -- altering the feeling, but not the root -- this exercise asks you to completely replace the existing chord with a completely different chord. It asks you to replace a character and try to retain largely the same overall play.
The function retained can be plot based or anything else.
Changing one of the characters in your play is difficult. Perhaps you'd grown attached to the domineering attitude of Blake the Unshaven Heathen and you're understandably hesitant to replace good old Blake with the agile Bambi the Brainy Barbie of Suburbia. (Or, perhaps it didn't work at all). But perhaps -- just perhaps -- you encountered a change in another character that led to something interesting. Perhaps you found a moment, a line, a scene, a feeling or otherwise that you'd can't imagine not including in your final draft.