When might it be acceptable to read a speech entirely from a sheet of paper?

The only time I can think it would be acceptable would be a complete impromptu speech.

Explanation:

Very often, if a speech or presentation is planned, you need to refer to your paper as little as possible, so that you can retain eye contact and have the speech go smoother (it gives you extra points with the audience as well). However, if it isn't planned, you can read off the paper. I'd still recommend giving eye contact as frequently as possible, so you don't loose the audience.

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