Doesn't ricin take days to kill, during which the agent can still be interrogated and rewarded with quick execution?
Ah, yes: "anywhere from hours to days", says Wikipedia.
Carfentanil, maybe?
Opiate overdoses are easily reversible with naloxone. I suppose it’s possible that carfentanyl is strong enough that it would take an unreasonable amount of naloxone to reverse.
Why not both carfentanyl plus insert fast but awful poison and hope nobody gives your spy naloxone in time.
Cyanide?
AFAIK cyanide is unpleasant; you die struggling to breathe because your muscles don't work - not because you became unconscious.
Thanks. I never knew that. I always assumed from movies that it was a quick and easy (hence painless) way to go.