Many speakers do this: practising in front of a mirror. So they can see how they ‘perform’. It’s a very logical way of thinking. But this thinking can be completely wrong. It can even be the exact opposite of what you want to accomplish.
Let me explain why you have to be careful rehearsing in front of a mirror.
1. It’s not real!
Even though you see yourself in the mirror, and the reflection is real, this is not reality. Practising in front of the mirror doesn’t reflect what happens in real life. Because for one, in real life, you don’t see yourself.
If you want to practice, you want to get as close as possible to the actual speaking experience. When you stand in front of the mirror you see a reflection of yourself, so your experience will not be the same at all.
Even more: you will not even see how you act on stage because you are focusing on your reflection. What you see in this case, is not what you get. This might also mean that you change things that don’t need changing!
2. You focus on gesture, not story
When you practice in front of the mirror, you are focusing on how you move. You will see every arm movement that goes wrong. You will see every little thing. This will make you focus on those things more. Which then leads to too much emphasis. And most probably it will go wrong. It’s like not trying to think about a pink elephant right now…
You want to focus on your story, not your gestures. In front of a mirror, you will lose the focus on the story.
3. It might make you nervous
Looking at yourself in the mirror when practising might actually make you nervous. When you look at yourself, you emphasise what goes wrong. You are much more aware of what goes wrong. Therefore increasing the likely hood of it actually going wrong.
If you see things going wrong in front of the mirror, you will start thinking about these things more and more. And that will make you nervous.
What to do?
Is it always wrong to practice in front of a mirror? I would say yes in most cases. But, like with everything, there are exceptions. If you practice early enough, it will most likely not make you nervous. And you can do that, but only to practice certain gestures. To see if certain gestures work or not. Otherwise, I would advise against practising in front of a mirror. Alternatively, you could record yourself. But I would not do this too fast, that might make you nervous again.
If you want to practice, do it in front of an audience. Find colleagues or other people to see you talk. They can give you feedback as well.