Threadhopping with this character - yes/no/it depends? Ask me first, please, unless the post specifies threadhopping is encouraged.
Backtagging with this character - yes/no/it depends? I love backtags! I will backtag for weeks.
Back-dating entries with this character - yes/no/it depends? Sure!
Hugging/kissing/other sort of intimate actions with this character? Ahah, go for it. He'll be startled though, and probably a bit awkward about it. If you try to hit on him, he'll either flat-out reject you (see: Jackie Tyler) or flirt back, depending on his mood/the situation/who you are.
Punching or causing some other form of physical harm to this character? Pretty much anything goes, just ping me if it looks like it's going to get serious (torture/death/etc.).
Is there anything that should not be mentioned near this character? Nine would like to avoid talking about uh, pretty much everything, but fortunately he is not in charge here. I say pry all you like~.
Is there anything you need us to know about interacting with this character (i.e. special physical features or abilities)? Nothing that Jchan hasn't mentioned about the Doctor already, I think -- blah blah telepathy, timeywimey stuff, etc. Oh, and he'll also have his
psychic paper with him. If your character has any psychic training at all, they may be able to see past the illusion. If not, they will see whatever the Doctor wants them to see.
Anything else that was not mentioned above? Not that I can think of at the moment!