I think Godmodding is never acceptable unless the person allows explicitly it. If someone wants an action out of my character, they can easily present a simplistic form of the scenario and ask "what would [my character] do?" Or invite me to write part of the post jointly with them. A simple 'yes/no' or generic statement might seem okay, but every character has the ability to respond in a unique manner, and when someone else writes for the character, it dilutes the uniqueness of the character.

I give exceptions for people with whom I've known for a long time and become accustomed to their writing style. There's one or two people that I know can manage a phrase or two and be fine, or add in some actions, and it doesn't bother me any. It's important to note that these people have the same level of character integrity protection as I do, and would have issues if any random Joe decided to write their characters for them.