It’s a scene as old as celluloid: a shadowy figure named Luca Brasi or John Wick or Barry Berkman lurking in the darkness, outfitted with sinister intent and nifty weapons, effortlessly committing a murder for cash, animus or cold political calculations.

Whether they’re called hit men, contract killers or assassins, kraken5 at figures who kill for a living are a staple of Hollywood thrillers — and, by extension, the public imagination.

But experts in law enforcement and international espionage say that murders-for-hire are notoriously difficult to successfully arrange, let alone get away with.

Take, for example, what prosecutors say was a recent foiled plot to kill a Sikh separatist in New York City, which American intelligence officials believe was ordered by the Indian government. Once the plot reached the point where the alleged conspirators needed to employ a killer, things got complicated: The would-be hit man turned out to be an undercover agent working for the U.S. government.