A few months ago I took off my jeans on stage in front of 600 Asian bankers.

This gratuitous exhibitionism was inspired by the supermodel Cameron Russell who once gave a TED talk in which she emerged wearing a skin-tight black mini dress that she proceeded to cover with a loose wrap skirt and a baggy sweater. Her point was that in order to talk as herself, she needed to dress as herself too.

My plan was to attempt the same trick, only in reverse. The purpose of my speech was to prove that “authenticity” at work – that trite cliché that has been seized on so uncritically by business people everywhere – was bunkum.

I decided to come slouching on stage dressed as myself, in jeans and a shapeless T-shirt, and then to change into the correct uniform for the sort of person who gives talks at banking conferences.

So on would go the smart dress and heels and off would come the jeans and Birkenstocks. The point would be that giving such talks is an inauthentic thing to do; to pretend otherwise is soppy.

Ill-advised though the stunt was, it taught me four things.

First, putting on clothes in public, as Russell did, is a walk in the park compared with taking them off.

Second, wriggling out of jeans without flashing your underwear is nigh impossible, even when you’ve got a dress on top and have practised the manoeuvre a dozen times at home in front of your mystified children.

And third, if all your energy is going into not showing your underpants, it is quite difficult to keep up a convincing patter about the daftness of authenticity at the same time.

Those three lessons, while all valuable, may be of limited use to public speakers in general. The fourth one, however, is universally applicable: when giving a talk your own performance is only half of it. The rest is down to a mysterious dynamic with the audience that starts in the first few minutes.

The experts on public speaking don't tell you this. It is nowhere in the new book Talk Like TED . This advises you to begin with a story, to be conversational, to engage all the senses, to try to be funny and to be brief.

Identical speeches
All are good tips, and mostly I follow them. But in my experience sometimes these tricks work and sometimes they don't. I have just given a series of five identical speeches to very similar audiences. One went really well, one really badly, the rest in between. The difference wasn't the material or the delivery. So what was it?

Each time I’ve found that it is something that happens at the beginning. If people start off laughing, they go on laughing. If they begin restless and bored, it is impossible to get them back.

Last week I was talking to a female chief executive who was puzzling over the same thing. She had just given the same talk on the same day to two large groups of her employees. One of the talks was a stonking success; the other was not. At first she couldn’t understand why. But then she worked out that the difference was two or three key people in the audience who set the tone for everyone else. At one talk these people were listening agog, in the other they looked bored and disaffected. An audience not only responds to a speaker, it responds to itself. When I’m listening to a talk and the person next to me is on her BlackBerry, I am much more likely to whip out mine. If she is feverishly scribbling notes, I listen harder.

Snorting and giggling
For this reason, I've almost given up sitting in audiences with my husband as his yawning and fidgeting is too infectious. When we went to see Black Swan in a packed cinema a few years ago, he let out a great laugh at the horrifying moment when Natalie Portman turns into a ballerina-cum-demon.

“Ssshh,” I hissed, but it was too late. A Mexican wave of snorting and giggling spread through the cinema.

This does not mean the reception of a speech is random, and that speakers should leave it to chance. Instead, what the jeans talk taught me was the inestimable value of a well-placed stooge or two.

The night before the talk I had dinner with the organisers. I warned them about the onstage outfit change and begged them to laugh and maybe clap. They looked a bit alarmed but said they would do what they could.

As I hopped around on one leg trying to free myself of the jeans under the gaze of a stunned and embarrassed audience, I cast a desperate glance at one of the stooges. She started to laugh and applaud and a couple of minutes later all 600 were at it. Disaster was averted, I recovered my balance and the talk was fine. I recommend the stooge trick wholeheartedly, though in future I will be leaving the clothes trick to supermodels.
– Copyright The Financial Times L t d 2014