Legally, how far can I go when quoting a public figure?

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I'm working on a product that uses quotes and information given by famous figures. Is there a limit on what I can directly quote from them? (Using what they've said in interviews and books?)

To avoid copyright problems, I'm going to use illustrations to depict the individuals rather than photographs. I figure it's in the same ballpark as 'unauthorised biographies' so should be legally safe but are there any other issues I should be aware of?

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