At the end of Avengers: Age of Ultron, it seemed as though Clint Barton, A.K.A. Hawkeye, had decided to retire from his career as an Avenger to live on his farm alongside his wife and children. And yet, he's still fighting alongside some of his old allies in Captain America: Civil War. According to Jeremy Renner, Hawkeye is back in action to "repay a debt" not just to Captain America, but to Scarlet Witch - seemingly referring to her brother Pietro sacrificing himself to save Hawkeye and a young boy in Avengers: Age of Ultron.
Here he is explaining the whole thing:
Captain America: Civil War hits theaters May 6.