The phone rings, late at night, in a deserted newspaper office. The voice is muffled but the aim clear: the mysterious mole has something explosive to impart.

That’s exactly how leaks to political journalists don’t happen, or at least not in real life. The business of extracting things from people who aren’t supposed to be telling you them is far more mundane than it looks in the movies, but also messier and more ambiguous and harder to pin down. That’s the crucial background to what might be called the Gavin Williamson problem, or the dilemmas thrown up by the sacking of defence secretary for supposedly briefing a Daily Telegraph journalist on discussions at a National Security Council meeting.

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Only Williamson and the journalist concerned, Steven Swinford, know exactly what was said during a phone conversation shortly before the reporter published his story. For all I know, they were discussing the weather. But what follows is as honest an account as possible of how, more generally, it feels to be inside the story sausage machine.

During a dozen years as a lobby hack, I can count on the fingers of precisely no hands the number of ministers who rang me up shouting: ‘You’ll never guess what happened yesterday in cabinet.” Equally, it’s rare for a journalist just to ring up and demand a rundown of cabinet, let alone anything as sensitive as an NSC meeting, and instantly get one. What did happen, week in and week out, was what’s best described as a process of jigsaw identification.

You might have got wind, say, of the fact that a controversial policy was going to cabinet that week. You might know broadly who was hostile to it and who in favour. So you’d start calling around. Generally, you’d keep the tone light, so that nobody felt terribly on their guard; just chewing things over, off the record. You might tell the minister that you’d heard there had been an argument in cabinet about X, and that they spoke up against X happening, and you just wanted to check before writing it up that that’s a fair description. Sometimes you would already know the answer. Sometimes you might be more or less guessing, or have a scrap of information and be fishing for more.

At that point, some ministers would say that they couldn’t help you with this one. Sorry, too sensitive. The smarter ones would try to find out how much you actually knew and how much you were bluffing before deciding how to answer. Very rarely, merely asking the question would unleash a torrent of information; more often, they would sigh and say, “All right, but you didn’t get this from me,” or answer in such a way that it was obvious that you were on to something. In a face-to-face conversation even the body language might give them away; if you know someone well enough, you can pick up when they’re uncomfortable, and at that point you know enough to bounce the story off someone else.

Facebook Twitter Pinterest Washington Post reporters Carl Bernstein and Bob Woodward broke the Watergate story in 1972. Photograph: AP

So, bit by bit, half-truth by guarded answer, you build the story. If they really wanted to a source could tell themselves that they weren’t leaking, as such. No, they were just reluctantly confirming something the journalist had already been leaked by someone else, or at least making sure that the details were accurate. When everyone else in government seems to be leaking, in a way that makes them look heroic, who wants to get screwed over by remaining dutifully silent? So, more often than not, the model for a juicy story is less Watergate than Murder on the Orient Express, with several people chipping in and no one entirely sure who dealt the fatal blow. More than once, I had someone tell me that they can’t imagine where I got wind of something, when the honest answer was, well, from you, or at least partly so. It’s surprisingly easy for someone to go fractionally further than they intended, especially when they don’t know what other pieces of the jigsaw a journalist holds, or else to convince themselves that leaking is something other people do.

None of this, of course, excuses what Gavin Williamson is alleged to have done. The presence of intelligence chiefs means a National Security Council is supposed to be inviolable, above the usual political game playing; to treat it like any old government meeting is to cross a sacred line. The message Downing Street has sent is that cabinet discipline may have collapsed under the pressure of Brexit, but that doesn’t mean anything goes.

But like anything people do in private, leaking is a complicated business in which the lines get blurred. For a while, the Williamson affair may well lead to people clamming up or becoming more guarded in conversation with journalists. However, if past experience is any guide, that won’t last long. I often wondered why people told me things they probably shouldn’t have and in many cases there are logical explanations, ranging from principled whistleblowing, via a weakness for gossip or a desire to build goodwill, to the narcissism of wanting to look important or the desire to advance a political agenda. With hindsight, I’m not sure they always knew themselves. After a while, perhaps it just becomes a way of life.

• Gaby Hinsliff is a Guardian columnist