Governor Tarkin was the Governor of the Imperial Outland Regions and the main antagonist of Star Wars: A New Hope, and one of the antagonists in Rogue One: A Star Wars Story.
Star Wars: Episode III - Revenge of the Sith
At the end of the last prequel film, a younger version of Tarkin makes a cameo appearance overseeing the Death Star I's construction.
Rogue One: A Star Wars Story
He appears as one of the antagonists of the 2016 film, commanding The Death Star, which Galen Erso had built for him. He at first tests his Death Star, and lets Orson Krennic choose the locations. After watching the functions of The Death Star with Orson, he was impressed of his new weapon. He and Orson however get into a brief argument, as Tarkin says he will prove Orson wrong. As Orson wants the weapon for himself, without Tarkin guiding him, Tarkin informs him there is a traitor from Galen's facility, pointing out to Orson that he is failing, and ordering him to kill the traitor. Orson goes to Galen Erso's research center. However, Galen revealed he had only built the Death Star so Tarkin or Orson wouldn't. Galen was killed during a battle in the research center. As Orson goes to his own research center on Scarif, Governor Tarkin who is informed that Orson is on Scarif, sends Darth Vader and his fleet to deal with the rebels, and prepares to extract the research which Orson extracts, including Galen's hidden messages, just as expected. He manipulates Orson into finding the hidden message of Galen's, as Orson does his job. Later, Tarkin realized he has to do things by himself, as he assumes that Orson had failed, as he expected. Therefore, he destroyed Jyn Erso, Cassian Andor and even Orson Krennic by shooting his weapon on Scarif and destroying it. Tarkin had later sent Darth Vader to get the tape with the message recorded on it, however, it was too late as one of the rebel soldiers have escaped delivered it to Leia first.
Star Wars: Episode IV - A New Hope
Introduced as the main antagonist of the fourth film in the saga, Grand Moff Tarkin is the Governor of the Imperial Outland Regions and commander of the Death Star. After the Emperor dissolved the Galactic Senate, Tarkin and Darth Vader are charged with pursuing and destroying the Rebel Alliance. He then orders Darth Vader to bring Princess Leia to him, so he can either destroy Alderaan with a copy of the death star or destroy the base of the Rebels. His plans work, and he orders Leia to reveal the location of the Rebel Base. She reveals it is on Dantooine, however, he decides to destroy both of his targets, and he manages to destroy Alderaan first. He later occupies the death star itself. In the film's climax, Tarkin refuses to evacuate, and orders everyone to continue the operation anyway. Thereafter, he is killed by Luke Skywalker, who destroys the Death Star with his spaceship, while Tarkin is still on board.