Bill Hader’s mouth and chin were coated with fake blood, so when he pivoted, astonished, he looked like a vampire startled in mid-snack. On this January night, Hader was in character as the hit man Barry Berkman, on “Barry,” the dark comedy that he co-created and stars in on HBO. In the scene being filmed, for the show’s forthcoming second season, Barry has run into a Rite Aid for supplies. He’s bleeding from a brutal series of events that stemmed from his botched hit on a Tae Kwon Do expert named Ronny.

The premise of the show is that Barry, a former Marine sniper, is desperate to stop killing; he moved to Los Angeles in a misguided effort to reinvent himself as an actor. So when he went to Ronny’s house he began by trying to persuade his target just to leave town for a while. Ronny, stoned and affronted, kicked him in the head, and after an ugly struggle Barry bashed his windpipe and left him for dead. Now, in the Rite Aid, Barry is shocked to discover that Ronny is in the next aisle, trying on a neck brace. Overjoyed, he resumes his cajoling, but Ronny aims another kick at his head. Hader ducks it and then rises back into the frame, transformed by outrage, as Ronny swipes a shelf-load of Band-Aids at him.

Hader, who was also directing the episode, happily called “Cut!” and turned to Daniel Bernhardt, who plays Ronny. “That was amazing!” he said. “Maybe let’s try one with just a little more rage-build.” He acted out Bernhardt’s part, escalating from recognition to fury as he lurched forward, his rangy frame somehow realigning itself in the other actor’s brawny image. Even his demonstrative wheezing sounded like Bernhardt, with a signature squeak to its whistle. “Got it!” Bernhardt said, wearing the expression people often wear around Hader: How did you just become me?

When “Barry” débuted, last spring, Hader’s protean qualities were well established, from eight years on “Saturday Night Live” and a gallery of vivid film roles. Hader’s face—lupine smile, wide blue eyes, and accent-mark eyebrows—is at once transparent and supremely controlled. On “S.N.L.,” his closely observed impressions were a linchpin of the show. His Alan Alda—handsy, cloyingly ingratiating—underlined Alda’s neediness, and his Al Pacino pinpointed Pacino’s complacent admiration of his baritone sax of a voice. Kristen Wiig, who worked with Hader on “S.N.L.” and has acted with him in four films, said, “Bill drinks human nature in.”

But Hader had profoundly ambivalent feelings about being on “S.N.L.”: he was terrified of live performance, and his goal has always been to write and direct films that he’s not in. As an interim step, he cast himself on “Barry” as an actor who can’t act, and surrounded himself with characters who embody aspects of his personality. Barry is pulled in one direction by Monroe Fuches, a facile blowhard who books Barry’s hits and works every angle to keep him killing, and by NoHo Hank, a timid, malaprop-prone Chechen mobster besotted with Barry’s lethal skills. He’s pulled in another by Gene Cousineau, the narcissistic coach of his acting class, and by his girlfriend, Sally Reed, a talented but even more narcissistic actress in the class, who encourages Barry to open up but can’t really do it herself. Hader wouldn’t have to dazzle as an actor—he’d be the dense nucleus at the center of the show’s fizzy electrons. Then he won an Emmy for his acting.

After filming Ronny’s rage-build, Hader watched it on the monitors. Inches from the screen, he mouthed Barry’s dialogue (“We can still do the plan—it just got off to a rocky start”), wheezed along with Ronny, swept his arm to knock the bandages off the shelf, and flinched as they flew at the camera. He was playing every part, including the audience. When Ronny turned from pursuing Barry to head-butt a store manager, Hader bent double, then reared up, still laughing, to emulate Barry’s twitchy walk as he sneaked away. His laugh, which resembles the high-revving cackle that begins the song “Wipe Out,” continued for nearly thirty seconds, until the crew was equally overcome.

Hader’s delight makes people want to elicit more of it; his crew members, who call him Billito, cluster around him like a Secret Service detail. He jokes around a lot on set, delivering dialogue from “S.N.L.” sketches, or driving over traffic cones in the parking lot in pretended confusion. “Feeling very comfortable around everybody, becoming a de-facto family, is an important part of it, because then I don’t mind failing,” he said.

Despite Hader’s insecurities, the first season of “Barry” got great reviews and was a hit for HBO, averaging 4.4 million viewers an episode. Its cunning, cat-footed plots drew people in but left them off balance. As Barry wrestles with an irreconcilable conflict—he can’t be both an anonymous detached killer and a recognizable hypersensitive actor—the show oscillates between the savagery of “Taxi Driver” and the histrionics of “Waiting for Guffman.” At one point, he executes a gentle friend of his named Chris, who got swept up in one of Barry’s bloody skirmishes and was planning to confess to the cops; the episode then cuts to the acting class performing articulation exercises—“Many men making much money.” After Barry sobs onstage at the horror of what he’s just done, Sally tells him, “Whatever you did tonight to get to that place, that’s your new process, O.K.?”

In the pharmacy, Hader set up a shot in which Ronny launches a jumping side kick, misses Barry, and goes tumbling through a set of shelves stacked with Huggies and Depends. He consulted with his assistant director—a two-finger stab to show the angle; a hasty creep to show the cameraman’s move—then began pacing as he thought the scene through, surfacing occasionally when he recognized a familiar face to wave from his knees: “Hey, buddy!” Last fall, working on the Paramount lot, he stalked an idea for so long that he found himself in an unfamiliar office, and had to call out, “Where am I?”

After rehearsing the missed kick, Hader turned to his co-creator, Alec Berg, and said, “Alec, funny?” Berg, a forty-nine-year-old comedy veteran who wrote for “Seinfeld” and is an executive producer of “Silicon Valley,” has a patchy gray beard and an air of broody vigilance. He nodded rapidly. A chop-socky action sequence built around a would-be pacifist was undeniably funny.

“You know who did this kind of thing so well?” Hader said. “Blake Edwards.”

“Much better than we will,” Berg said.

Three hours later, Hader looked spent. All week he’d been fighting a cold by meditating and gulping water. Last year, he got two flu shots: belt, then suspenders. As he hurried back to the monitors to look at the final shot of the tussle, he turned to Berg, deadpan, and declared, “Let’s just say it: it is like going to war. ”

He sank into his director’s chair. After a long silence, he murmured, “I hate the way I sound, that nasally voice, and I don’t like my slumped-over posture.” He gestured toward his image onscreen: “I clearly don’t know how to fight, and I don’t look anything like my stunt double, this virile-looking guy. I look like my dad. It’s, like, can my double just replace me in everything?” Everyone looked away, politely, as he stared at the monitor.