If you’re someone who’s engaged in training people how to be more effective on television, as I am, you’re always on the lookout for examples of people who’ve really dropped the ball when the cameras are rolling. Step forward Zac Goldsmith MP.

In case you missed Zac’s moment of glory, here it is:

Whatever way you cut it, Zac took what was a tremendous opportunity to confront the accusations (and insinuations) thrown at him and wasted it. And what makes it even worse is that, with the right approach, he could have left the Channel 4 News studio confident in the knowledge that he had taken the wind out of the sails of the investigation against him. So, what went wrong and what should he have done?

What went wrong:

• Zac failed to recognise that his status as an MP automatically raises questions about his honesty and trustworthiness (we live in the post-expenses age). Accordingly, the onus was on him to prove his innocence rather than on Channel 4 to prove his guilt.

• Incredibly, it wasn’t until 6 minutes and 20 seconds into the interview that Zac finally addressed the questions being put to him. As a result, the audience simply saw ‘another politician’ trying to avoid the questions being put to him. In summary, Zac looked evasive.

• At 6 minutes and 30 seconds Zac responds to the figures put to him by Jon Snow by saying: "The figures you’ve used here, I assume they are correct." Sorry, he assumes they’re correct!? If you want to dominate in a televised discussion like this you need to be in complete command of the facts.

• When it came to the posters, Zac was in an unenviable position. By any objective measure, and certainly to the ordinary voter, a poster which has the face of the parliamentary candidate, their name and a command to vote for their party could not be said to be a local election poster. Yes, when it comes to accounting for the expense of those posters every other campaign uses the same trick, but that’s exactly what the public will see it as, a trick.

• As for the jackets, this should have been a home run for Zac. The facts were clearly on his side, as they would be for the many politicians from various party’s who have engaged in similar activity. However, the jackets were brought up 8 minutes in, and Zac spent 44 seconds blustering rather than getting straight to the killer point (which he didn’t raise until 8 minutes and 44 seconds).

There are many other minor errors which could be highlighted, but the points above represent the major tactical errors Zac made during what could have been a successful interview.

What should he have done:

• Assuming what Zac says about Channel 4’s failure to allow him to appear is true, he was absolutely right to lead with that and ask for an apology. As mentioned above, because he’s an MP the audience will already assume he’s in the wrong. However, his claims against Channel 4 (if valid) offered the opportunity to raise doubts about the integrity of Channel 4 News and Jon Snow. If you can weaken your opponent in the eyes of the audience, then you absolutely should do so and do it as soon as possible. But remember, truth has to be on your side.

• That said, Zac spent far too long trying to do this. He should have made his points, reading from the papers in front of him (citing times/dates) to add authority to his claims, promised to publish everything online after the show for viewers to check for themselves and then actively welcomed the opportunity to address Channel 4’s accusations and insinuations head-on. All told, Zac should not have spent any more than a minute doing this.

• As Abraham Lincoln said: "Give me six hours to chop down a tree and I will spend the first four sharpening the axe." Zac should have spent much more time sharpening his axe. Preparation is the key to success in television interviews. In Zac’s case he should have made a point of knowing the details of his own claims, his opponent’s claims and the claims of some other MPs (for context).

• On television you need to pick your fights, and the poster discussion was not one that Zac was going to win. He should have dealt with it as quickly as possible, citing examples of some fellow MPs and then made clear that he’s happy submit himself to the judgement of the Electoral Commission. Sometimes you’re going to lose, and when that happens you need to limit the damage inflicted by that loss.

• When it came to the jackets, Zac should have seized the opportunity to land some seriously heavy hits on Channel 4. Rather than spend time blustering, he should have got straight down to business and made clear that, yes, the jackets were bought for the 23 day period, but they were also bought for the local elections, the European elections, the GLA elections and the next parliamentary election, and that it simply wouldn’t make sense to attribute the whole cost to his campaign and for subsequent campaigns to declare no cost whilst enjoying the full benefit. The stickers were specific to the 23 day period yes, but the jackets were not. Simples. The argument sells itself, but by the time Zac got round to making it, he’d already lost the audience.

In politics, one moment can destroy a career, and whilst Zac’s moment on Channel 4 won’t destroy his, it will certainly stay with him for as long as he remains active in politics.

Still, in fairness to Zac, at least he wasn’t as bad Peter Ward of the British Dental Association: