The impostor had all of the memories and experiences of Picard, but his behavior was different from the captain. He was more outgoing. He was flirtatious with Dr. Crusher and fraternized with the crew, buying drinks and singing. This raised the suspicions of the senior officers.
The Picard impostor moved the ship dangerously close to a pulsar, which caused the ship's shields to begin to fail. Commander Riker took control of the ship, despite the impostor claiming that he lacked the evidence to defend himself at a board of inquiry. The Picard clone had been testing how far his crew would follow him, even to death.