While the first joked that Clint Barton simply grew bored with playing golf during all that down Renner explained to Marvel UK & Ireland that the archer returned primarily for Scarlet Witch and Captain America. He said:
He had to repay a debt to Scarlet Witch, and Cap called in need, so he came out of retirement.
The part about Captain America needing him makes sense. Because Steve Rogers is on the run due to not following the Sokovia Accords and helping Bucky Barnes, a wanted man. He needs all the friends he can get, and luckily, Hawkeye was willing to oblige. However, the part about Scarlet Witch is less clear. During the Battle of Sokovia in Avengers: Age of Ultron, Clint inspired Wanda Maximoff to overcome her fears and help battle Ultra’s army. Later, Wanda’s brother, Pietro, a.k.a. Quicksilver, gave his life to protect Hawkeye from gunfire. Maybe Clint feels responsible for Pietro’s death, even though he himself was protecting a child, and to pay off this debt, he joins Team Captain America.
Whatever the reason, Hawkeye and Scarlet Witch will be fighting alongside Captain America, Falcon, Winter Soldier and Ant-Man as the main protestors against the Sokovia Accords, a.k.a. the new legislation passed by the world’s governments mandating that the Avengers and other enhanced beings be officially monitored. Their opponents will be the pro-registration Team Iron Man, which consists of Iron Man (duh), War Machine, Black Widow, Vision, Black Panther and Spider-Man. The main clash between the teams in Captain America: Civil War will occur at an airport in Germany, and needless to say, thing will get ugly.
Just like in Thor, The Avengers and Avengers: Age of Ultron, Hawkeye will be pulling out his usual bow and arrow tricks, but we’ve also seen in some footage that he’ll eventually be forced to fight Black Widow, his former S.H.I.E.L.D. partner/best friend. Hardcore comic book fans will also be pleased to see the classic combo movie of Hawkeye shooting the tiny Ant-Man out into the field, which we guess we’ll then lead to Scott Lang becoming Giant-Man. This won’t be the last time we see Hawkeye return to his action-packed life, as Jeremy Renner is confirmed to appear in at least one of the Avengers: Infinity War movies
Captain America: Civil War charges into theaters on May 6.