When you want to do an elaborate setting and playing multitude of characters and the one interested is just not doing any effort. Observe the following scenario.
Someone posts on thread, “I'm interested.” You send a PM with requirements you need before you get started, from asking what sort of character he want to play to method of RP. Four or five paragraphs.
“Okay, we will do this by PM.”
You state again what else he needs to do.
“I have to think about it.”
Enter the feeling of wanting to scream at your poor monitor who face you every day. In this RP I will be doing, I will be playing 3 minor characters from the start, one of which may be two different things depending on the other player's character. Then comes 5-6 other characters entering the story, at least half of which will all be different... depending on the other's character. This is stuff I need to know before it even begins! I have the plot down, but for the plot to work, the characters may be different depending on the other's character.
I think what I have posted on my request is enough to show this is not going to be casual. One line responses won't be productive. I'm scared.