Why does Buzz Lightyear act like a toy around people in Toy Story?

Early in the first Toy Story, Buzz Lightyear believes he truly is a Space Ranger. His struggle to accept that he is not actually Buzz Lightyear, Space Ranger is a key part of the first movie in the series.

So why does he act like a toy when people are around? Wouldn't he try to interact with them as a Space Ranger would?

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