Grand Moff Tarkin is a fictional male Human in the Star Wars series. The character was portrayed on screen by Peter Cushing. His allegiance / loyalty is to The Galactic Empire. Grand Moff Tarkin is the commander in charge of the first Death star. He is senior in rank to Darth Vader. Tarkin doesn't hesitate in ordering the destruction of Alderaan in the first film Star Wars IV - A New Hope in revenge for Princess Leia not telling him where the Rebel base is located.
When the Rebellion attack the Death Star, such is his confidence that the attack will fail, he declines an option to leave the base.
Peter Cushing was never shown full size in the film because he was wearing slippers because the shoes he had been given were ill-fitted. In reality, Peter Cushing was a very nice person at complete opposite to how his character was portrayed in the films. Carrie Fisher who played Princess Leia had remarked she found acting with Peter Cushing difficult because Peter was such a nice guy in real life and Tarkin was the opposite. ref: Peter Cushing.
The actor, the very venerable Peter Cushing died in 1994 and so was unable to appear in person in the film. The producers of the film was able to digitally put him into the film so well that you're unable to tell its a digital image.
Grand Moff Tarkin takes control of the Death Star from Director Krennic who had overseen the space station's construction. Grand Moff Tarkin is seen as a back stabber, someone who see to take credit for the development of the station himself. When the Rebel Alliance attack Scarif, Grand Moff Tarkin orders that the Death Star travel to Scarif to assist in the defence of the base.
When the Death Star arrives at Scarif, he orders the main weapon to fire at the main Imperial base on Scarif. The main purpose being to stop the Rebel Alliance from transmitting the plans but also to ensure that Director Krennic for whom he hates is killed and therefore Tarkin will have no challenger in his position to Emperor Palpatine who doesn't appear in the film only a mention.