About a year ago a friend landed a grand job running a medium-sized company. A few weeks in, I had lunch with him and he told me he was having a pretty grim time. Half his staff was mediocre, the culture was sleepy and, although he had come up with a plan to change things, he wasn’t at all sure it would work.

Last week we met again and I asked how it was going.

Great! he said. It’s all good.

I congratulated him on having whipped everything into shape so quickly.

He gave me a funny look and said the only thing he had so far managed to whip into shape was himself. He had worked out that the first thing you have to do as a chief executive is to learn to lie. Or if not actually to lie, never to give voice to four basic truths.

The first rule is never to admit to not enjoying the job. When you are in charge, you cannot say that you are struggling or stressed or having any negative feelings about the work at all. Instead, you must insist that the job is stimulating and going well and that you are entirely in control.

The second truth you are forbidden to utter is that you don’t like or rate anyone at all in your organisation. “My chairman is an idiot,” is something often thought but which must never be uttered. Even less can you disparage anyone who works for you.

Lay it on with a trowel

Third, bad-mouthing your company is out of the question. You are responsible for it and for everyone who works there and you have to be chief cheerleader at all times, however unnatural that feels.

There was John Cryan last week laying it on with a trowel as he insisted that Deutsche Bank was "absolutely rock solid". By the standards of bank chief executives, Mr Cryan is a straight talker (he dared declare in November that his bonus didn't make him work harder) but even he had to play the confidence game – banks are only ever rock solid when people believe them to be so.

There are only two times when it is possible to say something negative: when you can blame it on your predecessor (and then it’s open season) and when you have a plan up your sleeve for improving matters. Even then, though, such talk can be dangerous.

When Stephen Elop likened Nokia to a burning platform, he spoke the truth, one that was needed to frighten employees into action. Yet it was a disaster. Things continued worsen until Nokia was finally bailed out by Microsoft, which bought its mobile phone unit.

The final thing my friend has learnt never to tell anyone is that he’s not sure. As chief executive, you cannot say: “We are making this takeover/ restructuring/slashing costs but I’m not entirely sure if it will work.” Instead, you have to present every initiative as a no-brainer. Not only is doubt over strategy forbidden but admissions of self doubt are ruled out too.

Even though almost all chief executives privately admit to being riddled with it – and those that don’t are the ones we worry about – they must not ever say: “I’m not sure I’m up to this” for fear they will be taken at face value.

Possibly these four forbidden truths can be confided to a sympathetic spouse in the dead of night but that is as far as it is wise to go.

Any sign of non-enjoyment, or doubt, or lack of faith in the company or individuals is taken as tantamount to admitting that you are unfit for the job.

Tight-lipped through hell

This is all pretty constraining, especially given how hard it is to be a chief executive and how often it ends in failure. With hindsight, some former chief executives will in unguarded moments admit that the job was hellish but at the time there was only one thing for it: square their shoulders and lie.

According to my friend, the good thing about pretending to be having a great time is that it helps you convince yourself that you actually are; the bad thing is that it is thoroughly alienating. If you can’t tell your friends what you really feel, there isn’t much point in having friends.

As I headed back to work it occurred to me that his list was not complete. There is a further taboo and over this lunch he had comprehensively broken it.

That fifth thing you can never say as a chief executive is: “My job means that I sometimes have to lie and pretend.”

There is only one acceptable view of what it takes to be a great leader, which is that you have to be honest with yourself and others at all times.

This simply is not true. A chief executive can only afford to be honest up to a point. In fact, the scope for honesty is so curtailed that the job is quite unsuited to anyone who, like my friend, is too addicted to saying what they really think.

Copyright The Financial Times Limited 2016