Understanding Requirements for Ragdolls

[Brutal]

I have read over the Ragdoll Bullet Demo, and searched all of the Ragdoll threads here as well. Basically I am just wondering what is needed outside of Bullet to get the Ragdolls to work. I purchased a character model from a website a few days ago I am using as my main player model for the time being.

What I don't understand is if I need to have some sort of custom collision/physics mesh for this model for it to work as a Ragdoll? When I did modding for Valve games, I remember exporting models and you would have to generate a physics mesh along with it. From what I have read, I really haven't seen any mention of this but it seems like it would be necessary for a Ragdoll to function properly.

Thanks for any information