So you've found a thread which has the word "OPEN" in the title.
First thing to do, READ the thread. Let yourself know what your getting yourself in for.
A good thing to do next is to maybe PM the person who started the thread, find out what they wanted to achieve with the open thread. Maybe PM anyone else in there, see what they wanted to do, see if you can help to give them that achievement. If you have ideas, maybe its a good idea to you know, let the others know, see if they can help you.
There are many things to take into consideration how to join the thread. Is there an 'in' maybe they are at a hospital and it just so happens YOUR charrie is at the hospital. If a building is on fire, it could just happen that your charrie is IN the building, or walking past it.
Remember you are a guest in the thread. You should help the person to achieve what they want to... Unless you both have come to an agreement on what is going to happen to the characters.