People are always so optimistic in B-movies. Like in this sequence from 1954's Killers From Space, when they inject Peter Graves with truth serum and then the colonel says, "Oh, he'll make sense now!" — right before Graves launches into his crazy yarn about googly-eyed Groucho-browed monsters from outer space who brought him back from the dead. And showed him uncanny atomic calculations on the back of TV dinner foil. And made him watch a long montage about clouds and flames and cities in space, and daisies and ... wha, huh? Sorry, the drugs started wearing off.
Killers From Space is pretty much the zaniest classic scifi movie not to be subject to MST3K treatment. Directed by W. Lee Wilder, brother of Billy Wilder, it features aliens who kidnap a U.S. scientist and brainwash him into helping with their invasion plans. But then he regains control over his faculties after this whole truth serum incident, and manages to destroy the aliens by disrupting their power supply. You can watch the whole thing online for free at the Prelinger Archive. [Archive.org]