¤ backtagging: this is always ok! i will probably respond, assuming i don't miss the notif.
¤ threadjacking: this is always ok too. i am pretty flexible about this sort of thing!
¤ fourthwalling: plz no. isaac already has enough things on his mind without having to worry about being fictional.
¤ physical contact: no es bueno :( you are free to go for it, but expect a poor reaction no matter what you're doing.
¤ injury/death: ask first, plz & ty. i am usually down for whatever as long as there's prior discussion.
¤ sexytimes: man i ... only fade to black. sorry! so really this does not matter (and would probably never be applicable to isaac, even if it did)
¤ mindreading: this is generally ok ... just be aware that is it sort of a horrorshow in there. dead space is a series where a lot of awful things are constantly happening to everyone, always, and also an alien artifact is psychically influencing everyone around it to be crazygonuts. additionally, isaac is canonically diagnosed as having both post-traumatic stress disorder and paranoid schizophrenia; it isn't exactly a happy telepathy tea party in there is what i am saying. ALL THAT BEING SAID: if you ask me i will be more than happy to tell you what is on his mind in most cases 8)
i think that is everything?? will add more as is relevant, etc.