I just found a huge flaw with truth serum that would be hilarious to write about: You might have to tell the truth when asked a question, but WHICH truth is up to you. “I don’t want to tell you” would be perfectly valid, as long as it’s the truth! Or any variation of that!
“What are your feelings for me?” “A chemical reaction in my brain.”
“Where is the treasure?” “Somewhere”
“Is it on this island?” “Maybe.”
“What’s your plan?” “Well, its a collection of steps that will result in the desired outcome.”
“How do you construct this machine?” “Well, you take all the parts and a bunch of tools, and you use them to put the parts together.”
You can be as vague as you want as long as you aren’t lying! It’s the perfect way for your sassy/smart/clever character to become a pain in the ass of your villain!