Famous is, of course, relative. Some famous people are widely known, while others are highly distinguished in a specific area of expertise. If you find yourself in the position of introducing a famous person, this distinction can be subtle, but important.
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Generally speaking, widely famous people require briefer introductions. The phrase "our next speaker needs no introduction" has become cliche for a reason. For the most part, they don't. If it's done in an engaging way that quickly highlights the things they're known for, and sets up what they're going to say, you've done your job. If you know the speaker personally and have an interesting and relevant anecdote to share, feel free to briefly tell that story.
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For someone famous in a narrow arena, an academic who has published a highly acclaimed but not widely read work, for instance, more detail on their specific accomplishments are required. You could ask the speaker to draft the introduction for you, or, at the very least, provide the background information you need to write one yourself. Regardless, it's essential that you pronounce every word correctly -- even if it means spelling them out phonetically on the script from which you'll read. Your introduction should be no longer than a minute, two at most.
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