In the memoirs of Tsuboi, Sachio, an official in the Japanese colonial police, the author goes into some detail about Korean-Russians who infiltrated Korea to work as spies based on what he learned from suspected spies that had been arrested and interrogated.
He recounts the thorough training that the spies had to undergo before being dispatched to Korea. The majority were university students and usually entered Korea from the Soviet Union by an ocean route, landing on the beaches of Kangwon-do where police surveillance was thought to be relatively weak. They all went through a rigorous training regime on the outskirts of Moscow, under both Russian and Korean instructors which consisted of learning encryption techniques, operation of wireless radio sets, as well as learning the “Korean customs and common knowledge” of the day. This included making all of the spies memorize the oath known as the 皇国臣民の誓詞, recited at public events in colonial Korea, and the practice of showing a minute of silence for spirits of dead soldiers (英霊). When the spies entered Korea they carried nothing but Korean and Japanese made objects, usually used materials, and made to look as inconspicuous as possible.
However, Tsuboi claims, sometimes it was the little things that gave away the spies when they arrived in Korea:
Traps can be found in unusual places. In Korea relatively cheap imported tangerines from the Japanese mainland were sold, among other places, in the markets of the countryside and it was not unusual for the common people to eat them as an everyday food. However, in the Soviet Union at that time, apparently the average person had never seen a Wenzhou tangerine (( Another word for tangerine in Japan. Read more here. )) (温州ミカン) before. There was a case of a Soviet spy who had just entered Korea that, when being questioned, was offered a tangerine. The suspect bit into the fruit with its peel intact, as if one was eating an apple. (( in 坪井幸生『ある朝鮮総督府警察官僚の回想』草思社, 2004. p114 ))
While this story could well be apocryphal, perhaps passed around the office with a laugh in the way we circulate such stories by email today (but under far less sinister circumstances), it is an example of how incredibly challenging it can be prepare a spy for all eventualities. I have heard similar stories of Russian and North Korean spies being exposed for equally unexpected reasons despite having been given an incredible amount of training.
Tsuboi is understandably completely silent on issues of interrogation techniques and what sentences were given to convicted spies when their cases went to court but devotes a whole chapter to describing and justifying the widely used “illegal” technique of turning (逆用する）spies and using them to undercover a whole intelligence network.