Alien doppelgänger of Picard
This replica of Jean-Luc Picard was created by a group of aliens during their experiment to study Human leadership qualities. He replaced the real Picard on the USS Enterprise-D when he was abducted by them.
The replica had all of the memories and experiences of Picard, but his behavior was markedly different from the captain, as he tested various aspects of leadership. He was more outgoing. He was flirtatious with Dr. Crusher and fraternized with the crew, offering drinks and singing. He also ensured he had a new physical on record with Dr. Crusher, in anticipation of the suspicions of the senior officers. He ordered a seemingly needless engine efficiency increase from Geordi La Forge and casually asked Deanna Troi how far the crew's loyalty would go with him.
Diverting the Enterprise to study a pulsar, instead of their real mission, he tested the crew's resolve when he ordered the ship dangerously close to it, which caused the ship's shields to begin to fail. Commander Riker took control of the ship, despite the replica claiming that he lacked the evidence to defend himself at a board of inquiry.
When the real Picard was returned to the ship, his doppelgänger disappeared. (TNG: "Allegiance")
The Picard impostor was played by Patrick Stewart.