Let's not talk about finding your core message or bullet points for a moment, but look at the performance, i.e. things presenters do while they are giving their presentation in front of an audience.
Here are three things I noticed that the presenters themselves were probably not even aware of but that are somewhat irritating for their audience.
When you speak a lot, you get a dry throat. Some speakers can speak for a long time before that happens to them, others need to take a sip more often. That's not a problem at all. Something I notice quite often, though, is when a speaker feels the need to take a sip of water, grabs the glass or bottle while still talking - and then keeps on talking. And talking. And talking. And never gets around to actually take a sip, sometimes even putting the glass or bottle back down. And all the time, the audience was waiting for that pause or - worse - watching with some concern whether or not the speaker would spill the water as they gesticulate.
It's really only a minor thing, but it can be very distracting for the audience. The speaker starts an action but never completes it. As a speaker, check if you are prone to doing this (by watching a recording for a talk or asking a friend who attended). When you grab that bottle or glass, make it a habit to follow through and actually take a sip, then put it back down. Your body sent you a signal - listen to it.
Not using a remote
When you are using slides for your presentation, you should be using a remote control to forward your slides, period. Otherwise, you'll either be glued behind your laptop (or, worse, a lectern) or you'll be walking back and forth between your spot on the stage and your laptop. The former makes for a rather static performance while the latter is very annoying for the audience. If you are not familiar with using a remote, practise.
An odd case that I've witnessed recently was when a speaker was given a remote by the organisers - but most of the time, she forgot that she had it. She ended up advancing the slides by pressing a key on her laptop with the same hand that held the remote! Again, you need to be aware of these things before you can start doing something about it.
By the way, in the rare case that you have to present without a remote, e.g. because you forgot to bring it or it isn't working, remember that most slideware will let you advance the slides by pressing the space bar. It is much easier to hit than the arrow keys, so it's a little bit less distracting (for you and for your audience).
Not switching off the projector
Most talks use slides, so event organisers will often assume that you are using them and leave the projector running. So if you are presenting without slides, you'll end up with a projector displaying something irrelevant or even distracting (like a moving or blinking "no signal" notice). Consider switching off the projector if you don't need it. Alternatively, if there's a computer still connected to it, start any presentation and press the "B" key. In most slideware, this will switch to a black screen.
In a rather distracting case of not switching off the projector, I recently attended a talk where the speaker announced at the beginning that he wasn't going to use slides. The organisers had created a welcome slide with his name and topic on it, which was projected behind him. They hadn't actually started the presentation, though, so it was actually showing PowerPoint's slide view - and a Windows task bar. Which resulted in the speaker having the Firefox icon being projected onto his forehead for most of the talk.
And again, this is something that speakers need to be made aware off. He wasn't using slides, yet the projector was running and even shining into his face as he moved around, but it never occurred to him to do something about it.
As a speaker, you have to be aware of little quirks and habits like the ones described above. They can be very distracting for the audience, taking their attention away from your topic and thus have a negative impact on the effectiveness of your talk. I know it's painful, but do watch yourself in a recording of your talk (if there is one) or ask a friendly member of the audience to watch out for these things for you.
And last but not least: Don't be afraid to do something about the situation. If you don't need the projector, switch it off. If something is in the way, move it. If you need water, ask for it. Both the audience and the organisers are on your side - they want you to give a good presentation and if there's anything that can help you achieve that, they will be happy to help.
(Photo Credit: mass water crisis '10 by Elise, CC BY, from Flickr)
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