Anthony Mackie struggled to pile on the pounds to portray Martin Luther King, Jr. in new movie All The Way, branding it a "headache".

The Hurt Locker star had just finished production on Captain America: Civil War, in which he plays Captain America's superhero sidekick Sam Wilson/The Falcon, and he had been hitting the gym to tone up for the part.

However, instead of staying in shape, Anthony had to undo all the muscle work he had achieved so he could really become the civil rights leader for All the Way, the TV film adaptation of the hit Broadway play.

The 37-year-old admits the physical differences between the two jobs proved to be an unexpected challenge.

"Unfortunately, I did Captain America before I did All the Way," Anthony told TheWrap.com. "(Director) Jay Roach wanted me to gain a few pounds, so it was a little bit of a change. The weight was the biggest headache. I put on 10, 15 pounds."

The extra weight around his waistline turned Anthony into a target at home as he was teased about his heavier figure by his wife, childhood sweetheart Sheletta Chapital, who he wed in 2014.

"It got to the point to where my wife was calling me 'Fat Mackie,'" he laughed.

Anthony also had to head back to school to research his role, visiting King's alma mater, Morehouse College, to view and read his collection of writings. He also received plenty of reading material from Roach, which really helped him to develop his portrayal.

He explained, "The first conversation I had with Jay, he sent me tons and tons of clippings and writing and essays about Dr. King. I read this book called Bearing the Cross (by Pulitzer Prize-winning King biographer David J. Garrow). And Tavis Smiley wrote a book called Death of a King, which was a really amazing look at the last 365 days of Dr. King's life. It was probably my most important piece of source material."

All the Way also features Bryan Cranston reprising his role of President Lyndon B. Johnson from the stage show, and Anthony admits part of the attraction of the project was to set the record straight about the nation's 36th leader.

"All I heard (as a kid) was how Lyndon B. Johnson got us into the Vietnam War, Lyndon B. Johnson was a wasted president," he recalled. "Ironically, the more I learned about him and read about him, I saw he was a great president. The Emancipation Proclamation would just be a piece of paper if not for the Civil Rights Act of 1964. There was no president who did what he did in eight months. He was a phenomenal president and I think that shouldn't be overlooked."

All the Way aired on cable network HBO on Saturday (21May16).