Essentially, you take a certain agreed-upon number of characters from your WIP. Say 10. You agree on a certain number of words. Say 500 words.
Each of you tabs a personal file/writes in a notebook the numbers 1-10 and the names of the characters next to them.
Then you each make up scenarios to give the other.
Example: Writer One gives the following scenario to Two: Characters 2 & 1 are stuck in an elevator… and the song "Holla Back Girl" is on a nevernending loop. Meanwhile, Writer Two gives the following scenario to One: Character 1 finds fanart of them with enormous breasts. Apparently character 3 drew it…
The idea is that both of you are stepping into an unknown but with a goal, and through it, you might find some interesting quirks with your characters.
For me, it made me realize I was writing one story in an entirely wrong perspective. My friend said it helped her realize she was more comfortable with her adult characters than YA.
But we both had fun and found it useful and a neat way to get to know our newer characters/projects a little better.
Source: reddit post