Backtagging: By all means, yeeeeeesssssssss. Please don't hesitate to, if you want to.

Threadhopping: Go for it! Unless it's a locked thread, in which case probably not the best idea. Common sense!

Bending, breaking, or otherwise shattering the fourth wall: Yes, but again, common sense. Smallish things need no permission, but I'd rather not break his brain too soon into the game. Ask if you're uncertain.

Hugging/Kissing/Otherwise being touchy-feely: Yes, yes, and yes, but if you don't have his permission prepare for him to react accordingly.

Fighting/Injuring/Killing: By all means, fight! If you're going to injure, I'm probably okay with it as long as there's time to get him to medbay afterward. Nothing fatal, please.

Other: Use common sense. If it's godmodding, please don't do it. If you think you need to ask first, just ask. I'm pretty flexible.