I don't remember where it was but somewhere in the Internet somebody recommended to carefully and intensely learn any presentation you want to give. A point I absolutely agree with.

But a commenter answered that presentations after to much learning and preparation look stiff and like theater. I didn't get that. It never happened for me. What was he talking about?

By now I think I can guess what the commenter was referring to. If you 'learn' your presentation by memorizing exactly what you want to say, how you want to say it, how you want to move the result probably will look stiff.

Don't do that to your audience. Don't memorize your material! Memorizing is plain wrong.

Instead understand your material. Know your message. Know your own feelings about that message.

If your slides include a number that you can't remember without actively  memorizing it, that number is not important to you. If it is not important to you, you won't be able to convince the audience about it's importance.

Maybe the number is not important, but it's size, or it's relation to a different number. If that is the part that is important to you, you will have no problem showing that during your talk.

You want you company to head in a new direction? Envision yourself two years in that direction. How do you feel about that? Now envision yourself two years in the alternative direction. How does that feel?

Now show your audience your feelings. If you are emotional about a subject you will find the words and they will be way more convincing than any well crafted, memorized speech.

It still helps to practice your talk, but that is more for fine tuning your slides to your emotions, not the other way round