Regulus flinched. Not because he realized he was being mocked, but because he was insulted purely on an "academic level" (as he so preferred to refer to his snobbery) by the crude pronunciation of decor. He had almost made a comment on it, but he had lost the words at the back of his throat when the stranger had brought up a point of reason that he hadn't come to until now.
"...only one who can see me..."
So that meant.
When the realization finally did hit him, the weight of the world seemed crush Regulus in embarrassment. His jaw clenched, his ears burned hot and his stomach flipped over at the very thought that he had just made himself out to be a fool.
"I--well...it is not an assumption I made--daunted by the stupidity of your hideout and such." It was a poor defense, but it was the best he could do on such short notice and caught so off guard as he was. Though he aimed to look unaffected, Regulus' face did give him away; cheeks bright, bright red and he looked about as mortified as could be. "And how was I supposed to know that no one else could see you, hmm? There has been no one else to validate such a statement." Although, it still be safe to assume that anyone with a wand would use some manner of magic upon themselves.
Glancing down at the pavement, he grumbled some choice words under his breath and shook his head. "Initially I had suspected murder--what with all the--nevermind! There isn't any reason for a man to be dead one second and alive the next; lest you've found a manner of mixing your spells to give yourself the appearance of being dead and..." He trailed off, his mind automatically going through the lists and lists of potions, spells, herbs and whatnot, trying to decipher if any could be used to put a man in such a state.
He stopped a moment and looked up at the other, wide eyed and a little taken aback. "No. No, I haven't...I haven't got any." Regulus wondered if it would be worth the effort to continue pretending that he was muggle Thomas Williams or admitting that he was a wizard. Perhaps he could get away with calling himself a squib. He wasn't a very good liar, even he knew that, but he had to do all that he could to keep this stranger from realizing who he really was.
Being Regulus Black was incredibly dangerous.
"What's it matter to you anyhow?" He snapped, puffing his chest out and squaring his shoulders in an attempt to appear strong and domineering.