In fantasy, no problem. (Wordy as usual...)
Consentual tickling. Nonconsentual outside of fantasy is akin to rape in my eyes. Its much more fun when both 'ler and 'lee enjoy the experiance.
I agree, non-con in fantasy only. (Even there I feel a bit guilty...)
I wouldn't personally do this unless I felt sure the guy would regard it positively, and then I'd be cautious and do it,
if at all,
very briefly and privately so as not to embarrass him, as someone else wrote,
but I can only see a very quick, superficial non-con "surprise attack"
under those circumstances.
Certainly not more than a couple of seconds, and definitely not ever if the person is restrained or trapped, stuck in any way.
I probably wouldn't anyway, in case the person wouldn't welcome even that.
In any sort of "rape fantasy" or "force fantasy"
(obviously not possible, if it's rape/forced it's unwilling by definition)
the person fantasizes about someone OF THEIR CHOICE
doing WHAT THEY WANT DONE TO THEM,
so even if someone enjoys the idea of non-consensual tickling,
they may only want that with one person, or one sort of person, and only in a certain manner ----
---Otherwise it's an invasion of privacy, personal space, and could be highly uncomfortable, irritating or even painful, as others mentioned.
Since I don't know what (or who) another person would secretly want,
I wouldn't impose myself upon them.
Personal preference is a combination, from the considerate person of my choosing, more preferably from/to a "significant other."