In The Marriage of Alice and Frank, Lily mentions an evening spent impersonating Sirius under polyjuice. He spent the same evening impersonating her, or rather, her dark alter-ego from It Looks Like You Might Be One of Us. This little snippet is what he was up to.

Scenes from a War: The Perils of Becoming a Consummate Liar

Regulus stiffened as a dark-haired witch perched herself on the barstool beside him at the Leaky Cauldron, leaning forward and ordering a firewhisky (neat) as she sent flirtatious glances in his direction. She looked vaguely familiar, and on any other day, Regulus might have happily spent the evening practicing his charm on her, but he was a bit preoccupied at the moment. He was expected to go into battle (again) in just a few short hours – less than two days; had no bloody idea how to get out of it; and was waiting for someone altogether different in a last ditch effort to do so. In his desperation, he had arranged to meet with his estranged older brother, an auror and member of the Order of the Phoenix.

Well, arranged might be too strong a word. He had sent a letter telling Sirius he needed to speak to him, swearing in blood that no harm would come to him at this meeting, and included the place and time. He might not even show up, but if anyone could get Regulus out of the Death Eaters, it would be Sirius. Of course, Bella would try to kill him for deserting, and Mother would probably disown him, but he couldn't face another battle. He was certain he wouldn't survive it. If he could convince Sirius to arrange some sort of protective custody, he might have a chance. More than he would have on the battlefield, anyway.

The witch's drink arrived, and she sipped it slowly, with every impression of enjoyment, very obviously ignoring him. When it was nearly gone, she lit a cigarette with a click of her fingers and blew a smoke-ring directly into Regulus' face with a teasing smile, threw back the dregs and hopped off her stool, ostensibly headed for the loo. She paused at his shoulder. He could feel the heat of her body against his back, though he refused to turn to look at her.

"Rubbish disguise," she whispered delicately into his ear. Regulus straightened so quickly he nearly bashed his head into her – his – face. Sirius laughed, his barking chuckle not at all suited to the body he was wearing. "Care to head somewhere a little more… private?" he offered, slightly louder, before sauntering off toward the stairs.

That… bloody… Regulus suppressed his irritation and followed.

As soon as they reached the room Sirius had obviously rented for the evening, all premise of flirtation vanished. Sirius cast a bevy of security charms and wards, encasing them in what essentially amounted to a bubble of magic preventing any attempt at eavesdropping. Regulus added a few darker variations of his own, too. There was no such thing as paranoia when one was considering betraying the Dark Lord.

When they were satisfied, they simply stared at each other for a long moment. There was something odd about the face Sirius was wearing. The hair had the faint shimmer Regulus associated with a color-changing charm, and he was pretty sure that the glasses were enchanted. The eyes behind them were unnerving, uncannily similar to the Dark Lord's, back when he used to come around for holidays as de Mort. He wondered if Sirius had done that on purpose. "Whose face are you borrowing?" he asked abruptly, for lack of a better icebreaker.

Sirius smirked. "She calls herself Asphodel. I wasn't sure if you would have met."

Asphodel… why is that familiar? Regulus wondered. After a moment of awkward silence, it came to him. One of Severus' pet-names for the mudblood, Evans. "Did Evans enchant the glasses, or did you?"

"What? Why?"

"You look like de Mort. It's creepy."

Sirius laughed. "Don't let Trixie hear you say that. She might get offended on lover-boy's behalf. Anyway, what do you want, Reggie?"

The younger wizard swallowed hard. This was it, the moment of truth. "I want to defect," he said, too quickly.

"Pull the other one," his brother scoffed.

"No, Siri – truly! It's – it's not what I thought it would be like, the Death Eaters. The Dark Lord is… he's as mad as Bella and I can't do this anymore!" the last phrase came out in a rush of genuine fear, which Regulus was helpless to suppress.

Sirius/Evans' lips curled into a sneer. "You seriously expect me to believe Mr. I Want to Make the Family Proud, Mr. I've Wanted to be a Death Eater Since I was Fourteen, has cold feet about doing his sworn duty?" He closed the space between them with two long strides, wrenching Regulus' left sleeve up to his elbow. "That is until death and beyond, isn't it? You killed to earn it, didn't you?"

Regulus nodded miserably. "I – I hate it!" he admitted, voice breaking. "I hate killing. I hate dead people. I'm fucking terrified, Sirius! I – I just – I can't!" There were fucking tears in his eyes. He swiped them away viciously, unwilling to admit how close he was to the edge of a complete breakdown.

Sirius threw his arm down, and whirled away in disgust. "I can't believe you!"

"W-what?"

"Turning on the water works, like that would sway me? When was the last time that worked? When you were four?"

Regulus glared. "I'm not – Sirius! Dark Powers, would you listen to me?! Fuck! I'm not trying to – to manipulate you or something! I'm just asking you, as my brother, to –"

"Brother, is it, now?" Sirius laughed humorlessly. "I have three brothers, and none of them stood by while my own cousin used the cruciatus on me in front of the entire family!"

"I – That was years ago, Sirius! I was, what? Fourteen? Young! And stupid!"

"Youthful ignorance only excuses so much, Reg! Where were you when Walburga disowned me? Pleased as fucking Punch to be the new Heir of the House! Where were you when you last year? Still supporting your slimy fucking friends, cursing 'mudbloods' and 'blood traitors' in the corridors, getting yourself fucking marked! How old were you, Reg, when you begged him to brand you like his fucking property? What happened to 'a true Black never bows'? What happened to your fucking pride?"

"I – I was marked over Yule. And – and… I don't know, okay, Siri? I don't know! I just know that I can't keep doing this. I'm not like Bella – I can't just, just kill people – and I'm not like you – I'm not fucking brave – I need to get out! Please! Just – just take me to Dumbledore! I have information! I can – just keep me away from the Dark Lord and Bella, and I'll tell you everything!"

Sirius froze, a blatantly considering look crossing Evans' face. It looked entirely out of place, even disregarding the black hair and blue eyes: Evans had always been more subtle than that. And then a much more familiar, resolute expression fell into place. "No."

"N-No?! But! Siri – I'm begging you, please! Even if – even if you don't think of me as a brother anymore, please, take pity on me! Help me!"

The disguised wizard laughed cruelly. "Help you what? Become a spy? Infiltrate the Order? Pull another Blitz? Yeah, no."

"I –"

"I'll admit, you had me going for a minute there, Reg. You're good. I can't even tell when you're lying anymore. But I know you."

"I'm not lying! Sirius!"

"That's exactly what you would say if you were lying," the elder brother claimed, with an air of irrefutable confidence in his assessment. "You think I'm going to let you anywhere near the Order? Seriously? Just how big of an idiot do you think I am?"

Regulus was unable to stop himself making an inarticulate and undignified sound of frustration. "Are you fucking serious?" For once in his life, Sirius forewent the opportunity to make a terrible name pun, which was just as well, because if he had, Regulus might have been forced to hex him, and that would have gotten him exactly nowhere. "Your evidence that I'm lying is that I said I'm not lying?! Give me a little credit! If I were lying, I would never say something so obviously false!"

"Unless you knew I knew you wouldn't, so you did, to make me think you're being honest," Sirius smirked. "Walburga raised me, too, in case you've forgotten!"

"Of course I haven't, you stubborn – git! With that logic, there is absolutely nothing I could say that would convince you!"

Smugness sat well on Lily Evans' features, but it spawned a seething pit in the depths of Regulus' stomach. "No," Sirius said simply. "When you put it like that, I guess there's not."

"Why did you even come here tonight, then?" Regulus asked, voice trembling, trying very hard, now, to keep from actually collapsing into fearful tears. If Sirius wouldn't help him, he had no idea what to do. He was out of options. He was going to have to go through with the battle.

"Curiosity?" he grinned sadistically. "It has been, what, six years since you wrote me a letter? And it's been even longer since you've claimed a need to see me. I thought you might have had something worth saying. But, as it turns out…"

"You're condemning me to death. You know that, don't you?" Regulus asked. His voice sounded pathetically weak, even to himself.

Sirius sneered. "You've condemned yourself. You've always lived down to the expectations of the fucking family, always did exactly what they wanted; never had an original thought in your life, let alone acted on it. I honestly have no idea if you're lying to me or not, and quite frankly, I don't care. If you're not, well, you made your bed, Heir Black. I can't help you." He cut off his security charms with a violent slash of his wand, and brushed past Regulus roughly, shoving him off balance, even with his currently smaller frame.

"Won't. You won't help me. I'm going to die, and it will be on your head."

Sirius paused in the doorway. "No. Can't. I can't trust you, so I can't help you. And that is not my fault. I'm not the one who made an art form out of lying and manipulating and being whatever other people want me to be. You made your choices."

And I'm making mine, went unsaid.

So did goodbye.

Sirius walked away, footfalls echoing down the hall, no hesitation in his stride.

Regulus collapsed onto the bed, shoulders shaking, out of options. Truly, he had been out of options already: asking Sirius for help had been madness. Best to forget this episode had ever happened. There was nothing left but to pull himself together, go home, and prepare to face the prospect of kill or be killed. (And the bald fact that the latter was far more likely.)

After all, Sirius was right: he had wanted this. He just hadn't known when he was a child, wishing more than anything to be a part of the war, exactly what a war entailed.