It's always been a kind of interesting fit: Johnson generally plays a nice guy, has a hugely marketable smile, and rarely plays anything but the lead or co-lead in a franchise. Black Adam, traditionally, has been the villain in the Shazam! franchise, a dark opposite number to the Marvel Family who uses the powers Shazam gave him to avenge slights.
Johnson had been attached to DC for a while, with roles like Shazam, Black Adam, and Lobo being long rumored. He apparently had signed onto a Lobo movie, which fell apart before production got underway.
"I've loved the role of Black Adam," said Johnson, who had been rumored to play the character for years before it was made official. "I love that he starts off as a slave, that he felt like he was wronged. I've just loved that backstory. I think that Black Adam has always been, to me, the most intriguing superhero."
With Geoff Johns, an avowed Black Adam fan, at the head of DC's film division, it's interesting that Johnson and others continually refer to Black Adam as a "hero" or "antihero" rather than a villain. The likelihood that Black Adam is the actual star of the film and that the Shazam/Captain Marvel audiences might expect won't come up until a later film (if at all) seems to be pretty high.
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The Shazam! movie is still in the early stages of development; a few months ago, it was rumored that the movie could move up its release date from the original April 2019 schedule.