I absolutely cannot get this scene out of my head, so...
Anyone walking through the Zocalo would have noticed nothing more than two nice young men. They might even have spared a passing thought to just how nice they seemed; both of them were well-kept and smartly-dressed, a far cry from the usual riff-raff and shady sorts that were attracted to Babylon 5. Admittedly, the bow tie made one of them look a little old-fashioned, but there was nothing wrong with that. Two nice young men, smiling pleasantly and having a friendly chat. Nothing to draw much attention. Nothing worth talking about. Nothing worth a second glance. That's what anyone walking through the Zocalo would have thought...because they weren't looking at either man's eyes.
"So tell me, Doctor," Morden said, "what brings you to Babylon 5? What do you want?"
The Doctor smiled. It didn't make him look younger; far from it. The smile was the weary, cynical grin of a thousand years, entirely out of place on such a young face. "'What do you want?' That's a dangerous question. Possibly the most dangerous question in existence. Well, apart from 'What does this button do?' and 'Do you think I should cut the red wire first?' Both of which tend to get some remarkably shirty and unhelpful reactions--"
"How can it be dangerous, Doctor?" Morden smiled disarmingly. It didn't make him look younger, either. Morden smiled the same way that burglars jimmied open windows. "It's just a simple question. I'm trying to help you. And I can't help you unless you tell me what you want."
"But that's exactly why it's dangerous. When I answer that question, I give you information about what it is that I value. If I answer, 'Jammie Dodgers'--" the Doctor held up a hand quickly-- "just as a hypothetical example, mind you, I have some in my pocket already, they're a bit fluffy but still perfectly good--but if I do answer 'Jammie Dodgers', then you have a hold on me. If I have something you want, or something you need, you can get it from me by promising me Jammie Dodgers." The Doctor's tone was light, his voice rattling through the words as though they were in a hurry to leave, but his eyes were as cold and dark as galaxies.
"And since you know what I value, but I don't know what you want, you can make it seem as though your offer is highly prized...and mine almost worthless. Why wouldn't I trade whatever little trifle you ask for in return for my heart's desire? And before you know it, you're asking for a little more each time and giving away a little less, and I don't even realize the value of what I've lost and what you've gained, and I don't notice how much power you have over me until I've given away an entire universe...for a single stale biscuit." The Doctor leaned forward in his seat. His smile had vanished entirely. "A person who knows exactly what people want is the second most dangerous man in the universe, Mister Morden."
Morden stared back, his own smile still present but looking decidedly strained. "It sounds like you've already made up your mind, then," he said. "That's a shame, Doctor. I think my associates would have been very interested in you."
The Doctor looked to Morden's immediate left. Then his immediate right. "Oh, I suspect they will be anyway. People like them usually are. Assuming you have a very broad definition of the word 'people', which I generally do. I'm sure I'll be seeing you again, Mister Morden. Just like I'm sure you'll be seeing me." He rose to leave.
Morden let him get almost out of earshot before his curiosity got the better of him. "Doctor?" he called out.
The Doctor turned, looking as though he wasn't obeying quite the same laws of physics as everyone else did when he did so. "Yes?"
"You said the second." Morden's voice was very steady when he asked. It was the sound of someone trying very hard not to sound nervous. "What would the most dangerous man be?"
The Doctor smiled sadly, the sort of smile given by a teacher when a prize pupil made an expected mistake. "I should have thought that was obvious," he said softly. "The most dangerous man in the universe is a man who doesn't want anything you can provide." He turned away again, and his casual, "Goodbye, Mister Morden," was lost in the crowd.
Morden looked to his left. Then to his right. If he spoke, it was nothing anyone in the Zocalo heard.