At the risk of ruining everybody's fantasy, I'll chime in on this as a person who actually knows a lot of these models and has done this for about 25 years...almost all of them "fake" it to some degree now and again. The reason being, tickling isn't just physical, it's also psychological. I've hired models that were incredibly ticklish in a video I liked, only to find a very lackluster response. It's like sex. You can have bad sex, good sex and great sex. And it doesn't mean it's going to be the same every-time, even with the same person.The mood is a big aspect to it and if the model doesn't get in the "mood" you may have a very lackluster response.
If you want a peak behind the camera and see just how contrived most of your commercial tickling videos are, there is actually a Cal Star video out there with an out take that didn't get edited. In it, the girl says she's just not feeling it and tells the camera guy and the tickler to make faces at her and she'll "lose it" and start giggling and laughing.
It's a mix of art, in as much as it's something you're trying to create, and commercial. And even when there isn't a profit motive, the person filming it wants to put out the best video they can. So realism in tickling videos is a sliding scale. And you might be surprised at who is legit crazy ticklish and who is not. I won't name names, because it's not professional, but you get the idea.
My advice, don't look under the hood. You really don't want to see how it's made. Not if you already enjoy it. For the nit picker's, I can't help you. You're not only going to look under the hood, but start running diagnostics. I guess if it's your thing, but it just seems that it leads to endless disappointment. That is just my two cents on the matter.