To learn how Doug Williams acquired so many attributes as a quarterback – his arm strength, his tight spiral, his leadership, his resiliency, but especially his arm strength – you have to understand his primary job as a kid growing up in Zachary, La.

Without running water until he turned 14 years old, Williams was the designated member of his family to carry the house's slop jar, a nicer term for bucket holding excrement, which needed to be dumped into an outhouse nearby. To do this job effectively, Williams couldn't carry the bucket by his side – spillage onto his clothes was understandably a concern.

So, as Williams demonstrates in NFL Network's "Doug Williams: A Football Life," which premieres at 8 p.m. Friday night, he needed to extend his arm straight out, perpendicular to his body. According to Williams, it's how a scrawny and skinny quarterback could throw the deep ball with incredible ease and zip and get nicknamed "the Grambling Gun."

The documentary, which spends the majority of its running time focused on Williams' circuitous playing career and distinction being the first African-American quarterback to start in the Super Bowl, opens with a trip to Williams' old home, spawning memories and laying the foundation to understand the adversity that he's had to overcome in his life.

That's evident in the stories he remembers driving through his home state, remembering intersections that became cross burning landmarks and confederate flag parade routes. Those would inform the different kinds of racism and hardships (including losing his first wife to a brain aneurysm) he would endure as a professional athlete, even as he reached the pinnacle of the sport.

Split into several sections, the documentary chronicles Williams' four years at Grambling State University, where he set an NCAA record for touchdown passes and led the team to three conference titles, before exploring his tenure with the Tampa Bay Buccaneers after being selected in the first round of the 1978 NFL Draft.

Williams, with the guidance of head coach John McKay, took a sorry expansion team and turned it into a perennial playoff threat. After a contract dispute, he moved onto the USFL playing two seasons with the Oklahoma Outlaws. Eventually, head coach Joe Gibbs took a chance on him, offering him a backup role with the Redskins that would offer another round of adversity the next season, battling for the starting job with Jay Schroeder.

Those featured and providing testimony – teammates including Gary Clark, Jimmie Giles and Jeff Bostic, along with other people invested in his career such as Jon Gruden, Michael Wilbon and James "Shack" Harris – reflect on the struggles Williams had to persistently overcome. Williams downplayed the significance of his eventual achievement during the week leading up to the Redskins' Super Bowl victory over the Broncos, but admits in the documentary that the magnitude of the moment hit him on the sidelines.

Wilbon in particular remembers the pride he felt listening to broadcaster Keith Jackson end his postgame interview with Williams by saying, "Thank you, sir," evoking a degree of respect that Williams had strived to earn his entire life.

"Everybody has gone through something but Doug has had a lot of grief, a lot of really bad things that have happened," his current wife Raunda says. "And he has been able to use that as fuel and pick himself back up, and push through and succeed at the highest level."

With brief mentioning of his time later as Grambling's head coach and as a personnel executive with both the Buccaneers and Redskins, the documentary achieves its primary goal in explaining how Williams kept believing he belonged in a sport despite it throwing plenty of obstacles in his way.

"There is just some stuff you had to overcome," Williams says. "Find a way to get off the turf as coach [Eddie] Robinson used to always say. You have to find a way to get off the turf."

Doug Williams: A Football Life premieres at 8 p.m. on Dec. 7 on NFL Network, with repeat airings to be scheduled.