The Thick Of It cast were thrown in at the deep end when they filmed this weekend's inquiry episode.
Saturday's one-hour special features politicians Nicola Murray and Peter Mannion, as well as their spin-doctors Malcolm Tucker and Stewart Pearson, grilled under oath before the Goolding inquiry, set-up following the suicide of Douglas Tickel, a key-worker.
Producer Adam Tandy said that they filmed the BBC Two episode - directed by the show's creator Armando Iannucci - in a way which would make the inquiry, examining government leaks, as realistic as possible.
He said: "We gave our regular cast no rehearsal at all, and simply pushed them on to set with the cameras already running, and then our Clerk swore them in.
"They knew what they ought to say, but equally, they also knew what their characters couldn't reveal.
"It must have been hard to face the questioning of our inquiry team, who were under instructions not to give them an easy ride. Even when we stopped to do any retakes, there was no idle chatter allowed between the cast members. It was very much us and them.
"We even kept the witnesses away from everyone else before they appeared before the inquiry, just to keep up the pressure."