Just thought of something that humoured me.

Let's say you had someone tied to a chair, for whatever reason. Maybe he broke into your house, and you knocked him with a shovel from behind. And then you tied him up to a chair for questioning before the police came.

Like in the movies where a villain forces the tied-up hero an evil capsule, you pry his mouth open and toss a white capsule in and tell him to swallow it - maybe taping his mouth shut so he can't spit it out. But like any person would do in that situation, he'd avoid swallowing it at any cost. So, as you continuously yell at him telling him to eat it, he holds it in his mouth for as long as he can.

So, the kicker is that I'd use Vitamin C. Something harmless (even healthy!), but he wouldn't know better, thus agonizing at the sourness of the mystery pill as it slowly dissolves in his mouth.