At every turn, “The Shield,” which begins its long-awaited final season 9 p.m. Tuesday on FX, has defied expectations.

The first person to be surprised was the show’s creator, Rockford native Shawn Ryan. He assumed each season might be “The Shield’s” last, despite the critical acclaim and ratings success it enjoyed when it debuted on FX six years ago.

Well before that dazzling debut, however, Ryan got an even bigger shock, when FX decided to make his uncompromising pilot script in the first place.

“I didn’t think anyone would make it the way I was writing it. I thought maybe somebody might like it and make me change it to be network-friendly,” he said in a recent phone interview.

He wasn’t alone in that belief. (Now would be a good time to exit this story if you’ve never seen "The Shield" and plan to catch up with it on DVD. This piece does not contain spoilers about Season 7. The video clips in this story are from Episode 1 of "The Shield’s" final season. More video clips from Episodes 1-6 [and soon, complete episodes] are available here.)

Damon Lindelof, the executive producer of “Lost,” recalled reading Ryan’s well-regarded “Shield” script when it was making the Hollywood rounds almost a decade ago. As he turned the pages, he waited for elite cop Vic Mackey to be revealed as someone who resembled “NYPD Blue’s” Andy Sipowicz – “a good guy despite his gruff exterior,” Lindelof said.

That never happened. In the final moments of the first episode of “The Shield,” Mackey (Michael Chiklis) committed an act that he would spend years trying to redeem.

“Mackey murders an Internal Affairs rat in cold blood. He kills a cop. Shoots him in the head,” Lindelof said. “And when I read that, I thought to myself, ‘Shawn Ryan will never get this ending on the air.’”

“Well, I stand corrected,” Lindelof added.

Not only did FX shoot that shocking ending, Peter Liguori and Kevin Reilly, who were then the top executives at FX and who are now the heads of the Fox network, put Ryan in charge of “The Shield.”

It’s not uncommon for the person who creates a TV show to be elbowed aside once it goes into production, and “The Shield” was a critical project for FX ,which was then trying to put itself on the map with original programming. Despite his lack of production experience, FX trusted Ryan to bring the show to life. He was 34.

“I think one thing that helped me was, frankly, my ignorance,” said Ryan. “I had not done a lot of TV producing at that point. I had written for a couple of shows [‘Angel,’ ‘Nash Bridges’] , but I wasn’t involved in a lot of production decisions … I didn’t have any kind of preconceived notions about how these things were supposed to be done.”

Ryan credits fellow “Shield” executive producer and director Scott Brazil, who died in 2006, as well as the show’s first director, Clark Johnson, with coming up with the production strategies that gave the show its distinctive style. Money was tight: In its first two years, the show’s budget was $1.3 million per episode, which was “minuscule” compared to many network budgets, Ryan recalled.

“We didn’t want the show to be 85 percent of a network show, even though we had 85 percent of the budget,” he said.

When it arrived, “The Shield” certainly looked like nothing else on TV—except, perhaps, the more adventurous documentaries on HBO and PBS.

“The Shield” was shot in an immediate, fluid style, often with handheld cameras. Many scenes were filmed in hardscrabble locations in and around the show’s scruffy Los Angeles studio. As they tried to keep the peace on violent, gang-controlled streets, Mackey and his fellow Strike Team members found themselves in corner stores, in messy apartments, in claustrophobic interrogation rooms and in innumerable alleys.

You couldn’t count on tidy endings on this densely plotted show – thank goodness – but you could always count on the fact that Mackey and his team would run down a litter-encrusted alley at some point. Thanks to the show’s “you are there,” kinetic shooting style and energetic editing, you could almost smell the stench coming off the garbage.

“A lot of other shows have a tendency to try to glamorize the characters,” said Laurie Holden, who has a recurring role in Season 7 as federal agent Olivia Murray. “Working on this show, they like it if you have circles under your eyes.”

Mackey and his crew work out of a battered precinct house nicknamed the Barn, and on a July 2007 visit to that set, during filming of the final set of episodes, the show’s commitment to battered realism was much in evidence. Holes festooned the carpets, paint was peeling off battered walls and everything looked dirty. When a cup of coffee was accidentally knocked over, crew members grinned at the attempts to clean it up.

“I wouldn’t worry too much about that,” one said.

Despite, or perhaps because of, its unconventional depiction of life in the big city, “The Shield ” got noticed, and not just by awards-giving organizations and critics. In the 2003 and 2004, Ryan said, Brazil got calls from other TV producers and executives, who wanted to know how they made a show that won Michael Chiklis an Emmy and garnered truckloads of praise, all on a cable-TV budget.

Ryan has a deep-seated aversion to singing his own praises (compliments are usually met by attempts to give most of the credit to his “Shield” colleagues). Yet he does allow that “The Shield” did play a role in helping to create a bolder and more risk-taking aesthetic, especially in the cable realm.

“It wasn’t solely because of the creative success of ‘The Shield’ and [FX’s] ‘Nip/Tuck,’ but a lot of it was the economic success of ‘The Shield.’ It inspired people to get into this [original series] business, because we were making the show for a price that made sense,” Ryan said.

But the show’s budget dictated several deviations from the usual TV formulas. Complicated lighting setups took too long to set up, so artificial lighting was kept to a minimum. Shooting in small rooms necessitated the use of small, handheld cameras. Filming on location was routine.

“24” and “The Wire,” which premiered a few months before and after “The Shield,” respectively, as well as later series such as “Friday Night Lights” and “Battlestar Galactica” helped make that kind of “on the fly” aesthetic popular. But “The Shield’s” lasting influence can be seen in the popularity of its various directors, who are now working on everything from “Dexter” and “Saving Grace” to “Law & Order.”

“But that’s the nature of TV and film. It’s not the same as copying a test in school,” Ryan said of the influence of “The Shield,” which, he noted, owes a debt to everything from “The Sopranos” to “L.A. Confidential.” “I can’t say I was lying in the weeds, waiting to change television. I just took the circumstances I was in and did the best I could with them. And I think other people in my same position would have found similar ways to do things. I do like the fact that we were sort of the first to do it.”

For the first episode, Ryan trusted the vision of Johnson, an actor who’s both appeared in and directed episodes of “Homicide” and “The Wire,” but Ryan said he did have one request when the pilot was being shot.

“He and I had both been on police ride-alongs, him as research for being an actor on ‘Homicide,’ me when I was a writer on ‘Nash Bridges,’” Ryan recalled. “I just said to Clark, ‘I just want the viewer to feel like they’re on a police ride-along.’ And that’s what he brought to it.”

Perhaps the only similarity between Ryan and Mackey is that both work best when underestimated. When “The Shield” pilot was being shot, FX was also filming a pilot called “Dope,” starring Jason Priestly as a drug addict. Ryan had heard rumblings that that the network’s executives might be leaning toward picking up that show instead of “The Shield.”

“Not being the favorite helped us,” Ryan said. “We were able to make the pilot in such a go-for-it style that we eventually won the day, I think, on execution. I think our director out-directed their director. I think we out-cast their show. They had Jason Priestly playing a drug addict. And that seemed like a casting coup at the time, because nobody wanted to put themselves in an FX show.”

Few in Hollywood thought Michael Chiklis was right for the role of a volatile cop. At that time, Chiklis was best known for his much milder role in “The Commish.” Nobody could see back then that his star turn as the compelling Mackey would eventually inspire the creation of dozens of copycat bad boys, TV antiheroes with lots of flaws but little of Mackey’s depth.

But Chiklis, who lost weight and shaved his head before starting work on “The Shield,” brought a ferocious vigor to the role, which won him an Emmy in 2002. But the show’s intensity takes a toll, Chiklis said in a 2007 interview on the show’s set. (A longer version of that interview is here.)

“Some roles are exhausting physically, some are exhausting mentally. Some are really devastating psychologically. [Vic] is all of that,” Chiklis said. “You ever take a face cloth and soak it and wring it out? That’s me, at the end of the day. I’m a wrung-out washcloth. An overcooked noodle.”

As cast member Walton Goggins, who plays Strike Team member Shane Vendrell, noted in a separate interview, it can be especially difficult to work when the show’s shooting on location. Consecutive days of shooting in the grimier areas of Los Angeles, especially in the summer heat, are not unusual.

“An inordinate amount of focus is required to do our work on this job. And it’s not just the words, it’s not just the density and the complexities of the story. It’s the traffic. It’s the noises on the street,” Goggins said. (My interview with Goggins is here.)

Part of the reason for that exhaustion at the end of the day springs from nature of the show’s storytelling, which mixes complicated plots with the expert excavations of moral grey areas. Vic Mackey spends most episodes trying to escape the consequences of his canny but shortsighted actions, and that constant juggling act takes a toll on him and everyone around him. But the genius of “The Shield” is that it’s impossible to think of Mackey as purely evil. He’s convinced that he’s doing the right thing.

“Really, it harkens back to that Jack Nicholson speech in ‘A Few Good Men,’” Chiklis said. “He is the embodiment of that argument: ‘I am the man on the wall, you people who sit in your suburban households can look at what I do from a distance, and go, “Oh, isn’t that terrible, isn’t he awful?” But you can’t handle the truth of what we need to do to protect ourselves.’”

Though the show’s distinctive style and characters have been imitated – sometimes skillfully – on dozens of network and cable shows over the years, few have come close to making entertainment that creates such suspense within such a tangible, complicated world. After seven seasons, the cops in the Barn – Dutch (Jay Karnes), Claudette (C.C.H. Pounder), Shane, Ronnie (David Rees Snell) and Billings (David Marciano) – seem utterly real. It’s hard to picture the TV landscape without them.

“One thing I learned from Joss Whedon that I blatantly stole was the idea of approaching the stories first from character,” said Ryan, who is also executive producer of “The Unit” and has several new projects in development, including a private-eye dramedy for FX, a drama he’s working on with novelist James Ellroy for A&E, a sitcom for Fox and an adaptation of the book “Confessions of a Contractor” for CBS.

“The cop stories were always the last thing we’d figure out on a ‘Shield’ episode,” Ryan continued. “You just start from the premise of, ‘What do you want to happen to Vic and Shane this week? What do you want to happen to [fellow cops] Dutch and Claudette or Danny and Julien, or [Vic’s ex-wife] Corrine? What hoops do you want them to jump through?’ And then you figure out, what are the best stories that allow you to do that.”