‘The Blacklist” has been full of pleasant surprises for NBC this year. One of the most unexpected aspects of the Sony Pictures TV drama — the freshman success story of the season by any measure — is how much the series has been able to do with guest stars in the villain-of-the-week slot.

The show’s conceit demands a strong presence in each episode for a guest actor. James Spader’s Raymond “Red” Reddington is a master criminal now helping the FBI catch crooks — although to what end is part of a larger storyline that drives the show.

“Blacklist” blends the mystery that envelopes its main characters with a specific puzzle for Red and his youthful FBI liaison, Liz, played by Megan Boone, to solve each week. Red’s long career as “the concierge of crime” has seen him tangle with all manner of high-level bad guys and gals, some with exotic aliases such as the Stewmaker, the Alchemist and the Freelancer.

The casting of guest shots has ranged from established names (Alan Alda, Isabella Rossellini, Dianne Wiest, Jane Alexander, Lance Reddick, Andrew Dice Clay) to troupers deemed perfect for the part (Ritchie Coster, Tom Noonan, Ryan O’Nan, Justin Kirk, Hoon Lee, Robert Knepper). NBC has milked the promotional value of having Spader spar with a new nemesis every week via online video and social-media teases.

NBC Entertainment prexy Jennifer Salke credits the skill of exec producers Jon Bokenkamp and John Eisendrath for keeping the characters compelling and the adrenaline high.

“Once we saw how interesting the bad guys were every week, we saw that it was a way to tell more of Red’s story through the twist of the people he was up against,” Salke says. “Because James’ performance is so great, we’re getting great actors interested in doing a spot and playing against type.”

Dawn Steinberg, exec VP of casting for Sony Pictures TV, stresses that some roles call for a less recognizable face than say a Rossellini or an Alda. “ ‘Blacklist’ is one of those rare network shows that has a little bit of a cable feel to it, and we reflect that in the way we cast it,” Steinberg says, noting that traditional broadcast TV focuses on big names above all else.

With some characters, “you don’t want somebody who is famous enough so that it takes you out of the scene. There are some actors we choose to add authenticity to the story — so that you’ll believe that this is a person Red could have come in contact with years ago in China or Russia or Yugoslavia,” she says.

The chance to work with Spader is a selling point to prospective guest stars. His presence has a way of elevating the work even for guest thesps who don’t have any scenes directly with him, according to Ryan O’Nan, aka the Alchemist.

“He’s always kind of lurking in the background, and you want to make sure you’re up to his level,” says O’Nan, who adds that he had a sense “Blacklist” was going to click, based on what he observed on the Gotham-based set.

“I’ve been on other shows that didn’t feel as well-oiled,” observes the actor, whose character didn’t survive the episode. “It didn’t feel like they were still trying to iron things out. It could have been the fourth season from what I could see.”