If you’re having a disagreement and asking someone if they’re insane, you can’t be mild-mannered about it. You need to say the word “insane” as if you’ve spent time in a mental ward and know the meaning of the word. You can’t say “insane” like “tomato” or “laundromat” or “Allen wrench.” You need to get into it. One good example is the way Julie Hagerty says “you’re insaaaaane!” to Albert Brooks in the opening scene in Lost in America. You really have to go with the “aaayyne” part. You have to really bray it out.
