Backtagging: 100% Yes.
Threadhopping: Yes.
Fourthwalling: Let's discuss it first.
Romance/Relationship: I'm about the friendships and if they develop into anything more than that then it's a-okay with me. You can touch base with me if you need a little bit of ooc chatter about it first.
Physical Violence: If it makes sense in a thread then we can go for it, I'd probably just appreciate a bit of discussion before hand.
Mental/Magic: It would depend on the character and how strong they were, so this is definitely something that needs to be discussed in advance.