Author's Note: Tried to straddle Luthor's movie characterization and comic characterization. Might or might not have worked.
Some in-character Superman-bashing - about at the level displayed by various characters in the movie.
"So! Bruce Wayne." Lex Luthor clapped his hands together. "Mind if I take you aside for a minute?"
If Bruce was a decade or two younger, he might have smiled and come up with some polite excuse. Politeness had gone out of him when Robin... "Actually, I do," he said, and pulled away. Luthor grabbed him by the arm - well, he wasn't the only one without politeness - and gripped him firmly.
"I really think you'd like to hear this, Bruce - Can I call you Bruce? Of course I can." Insolence gleamed in Luthor's eyes. "I have some information you'd find very interesting. About, mm, a mutual friend of ours." His glance began to slide off to the side, then snapped back to Bruce. "Just us businessmen, right?"
What the hell was this? If Luthor seemed more violent, he'd think Luthor was trying to get him alone to kill him. But Luthor seemed too neurotic to dirty his own hands, and in a crowd... Bruce looked around the party. "How about we just go off into that corner?" he suggested, jerking his head that way. Luthor grimaced, then shrugged.
"It doesn't make any difference anyway. We might be - oh - on the other side of the moon, and it still wouldn't make any difference."
Bruce quirked an eyebrow at that, then moved to the corner. Luthor followed. The neurotic younger man looked both ways, positioning his back to the party, and then rummaged in his pocket before handing Bruce a crumpled note.
You were on the scene for the aliens' clash. You saw what they did to Metropolis.
One died. One remains. Are you for him, or against him?
A god in the sky - a god that ruined, a god that killed, a god that had no equal, no accountability, no weakness... He shut his eyes against the memory of the collapsing buildings all around him. "Against," he ground out.
"Not so loud," Luthor said dryly under his breath, and Bruce felt another note being pressed into his hand. He opened his eyes and read it.
I am opposed as well. As the wise man said, after all, "If God really existed, it would be necessary to abolish him."
And abolish him we may. I have obtained access to the dead alien's body and believe I may have discovered a weakness in its physiology. One available in only a limited quantity, but enough to construct a weapon.
I also have discovered that the alien likes to play human from time to time. On a hunch, I ran pictures of the alien against DMV databases and got a hit. Funny, really. By the time you receive this note, you might even have met him.
Bruce blinked rapidly and raised his eyes to Luthor. "Who?" he ground out, keeping his voice low. Not that it might help... Had Luthor been implying the alien might be at this very party?
"Mm, well, funny story, really," Luthor said, clapping his hands together. "You can tell a lot about a man by the way he shakes your hand..."
But what did that have to do with...
Bruce Wayne, meet Clark Kent!
No. No. No, that... The reporter? That nebbish, nerdy...
And there had been a time when the playboy act really was an act, wasn't there? When he'd played all of high society for the fool to keep up his own act, to disguise that worthless, harmless playboy Bruce Wayne was, in fact, a deadly, disciplined vigilante, a man who felt just as much contempt toward all of them as they did toward his mask...
His heart thudded in his chest like it did on a mission. Crumpling the notes in his hand, he shoved them into his pocket and gave Luthor a single nod.
Luthor flashed him a final, short note: Will you join me?
He thought of the monster moving among them, restrained by nothing, already half-worshipped by the fearful masses, laughing at them all behind its false humility... A monster that could kill everyone at this party before they could blink... A monster that could melt steel beams with a single glance, could tear apart a skyscraper like a child breaking a toy, could level a city without so much as earning a scratch...
He nodded. Luthor gave him a tremulous smile. "One question," Bruce said curtly. "Why me?"
Luthor paused, then raised an eyebrow. Pulling yet another note out of his other pocket, he handed it to Bruce.
Speaking of identities: you thought no one ever noticed that your ward tragically passed at the same time the Joker claimed responsibility for Robin's death? Or that your notorious self-destructive spree's start coincided with Batman's retirement, and its ending preceded his return?
Bruce raised his head. "You wouldn't have been that old then."
Luthor shrugged. "What can I say? I was a fan."
Author's Note: There's not a mentioned retirement interval in the movie, but it borrows heavily from The Dark Knight Returns, where Bruce permanently retired after Robin's death. I figured that I could split the difference by having Bruce temporarily retire at some point in the past, suffer a downwards spiral over his guilt, and eventually pull himself back into the saddle with a vengeance. Movie!Lex is crazy enough that he could just jump to conclusions based on Wayne's ward dying at the same time as Robin, but I thought this way would give him a bit more justification.
(Yes, Lex really is good enough to anticipate responses... Assuming Bruce would give the right answers, that is. He would have pretended to be friendly to Superman if Bruce had answered he was "for" Superman instead, but he'd have shifted his plans to something more resembling the canon movie.)
Hope this was interesting.