Out of Character:

Backtagging: Certainly!
Threadhopping: Most likely fine, unless clearly unreasonable.
Fourthwalling: Probably unfeasible, but we can talk about it!
Offensive subjects (elaborate): Nothing!

In Character:

Hugging this character: Go ahead!
Kissing this character: Go ahead!
Flirting with this character: Not stopping you!
Fighting with this character: Alfons isn't much of a fighter -- not physically and not verbally -- but if it comes up in a plot, we'll discuss it! It's feasible.
Injuring this character (include limits and severity): That's fine, as long as we've discussed!
Killing this character: Depending on the situation, perhaps! Let's discuss first.
Using telepathy/mind reading abilities on this character: Sure, but we may need to set it up.

Warnings: Nothing!