When you are talking about what someone does on a particular occasion, act and behave are interchangeable: ‘Passengers who left the flight in Rhodes said that they had seen two men acting/behaving very suspiciously.’ When you are talking generally about what someone does or what people do, use behave: ‘You can’t expect all babies to behave the same.’ ‘From the way he behaves, anyone would think that he doesn’t get paid.’
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