A random idea I had, mostly because the idea of the costumes amused me. :D
Matt doesn't know if he helped Light because the probably-fake memories told him they'd be friends someday, or if he paved the way for them to become friends by helping. It's one of those chicken-or-the-egg time-travel paradoxes that he can't help but be delighted by. But he's still contrary enough that the main reason he does nice things for Light these days is because Light's almost offended that Matt would presume to.
So Matt grins to himself as he hunts around for green tea in the kitchen cupboards. He prefers Red Bull, but he'll drink a cup of the tea. It's amusing to have an excuse to make it, all the more so because Light will see right through the pretense.
And, think of the devil, he comes in before the tea's done steeping. Matt looks up from Unseen Academicals and half-smirks, and Light glances from him to the teapot and shakes his head.
"Give it, like, another minute," Matt says.
Light lifts the teapot lid, and obviously can't find fault with anything, because he crosses his arms and doesn't comment.
Matt won't go so far as to pour the tea for Light, but he does wait to smoke until he's left the kitchen with his own little cup.
**
"That doesn't make us friends either," Light says the next time Matt passes him in the hall.
"I think you've entered the land of protesting too much, man," Matt tells him, opening the door that really ought to lead to the library.
Light follows him in, but whatever his comeback might've been, it goes unsaid: this isn't the library.
Matt's suddenly wearing Bermuda shorts, flip-flops, a Hawaiian shirt, a safari hat, and a very large camera around his neck.
Light has pointy shoes, a pointy hat with WIZZARD spelled out on it in sequins, and long burgundy robes. "I blame this on you," he says.
"What makes it my fault?" Matt objects.
"Because, Matt," Light says, with exaggerated patience, "your mind is the one full of useless things."
"But it did both of us. And..." He snickers. "You don't see a certain logic in it?"
"In that you're exceedingly hapless and probably made it as far as you did through sheer dumb luck?"
Matt's fairly immune to these insults by now. "You don't think you have pre-emptive karma?"
That just earns him a flat look from Light.
Matt searches the various pockets of the tourist getup, and finds a gold coin and a small phrasebook, but no cigarettes. "Damn," he mutters. "You know, we might be the only two people here who know who we're supposed to be."
Light sighs, looking extremely put-upon. "Eternity sounds less bearable by the moment."
"Dude, are you trying to convince me or yourself?"
"I'm sure I don't know what you're talking about, Matt."
He hasn't missed that Light hasn't maliciously used his real name in quite a while. "Who actually tries to get along with you here, other than me? Sayu, a Misa or two?"
"Oh, I see the error of my ways now. How grateful I am to be trapped in a sadistic mansion with you."
Matt has to laugh. "Man, I hope you stay grumpy about this. Years of entertainment for me."
"Is that a clumsy attempt at reverse psychology?"
"Nope. I totally mean it."
"Wonderful. You have even less depth than I imagined."
"You know, Light, just 'cause I'm not sneaky doesn't mean I'm not smart."
"Believe that if you must, Matt."
**
The next time Matt goes into the kitchen, there's a copy of I Shall Wear Midnight beside the teapot, with a note sticking out of it. He knows before he opens it that it'll say, in print so tidy it almost looks machine-made: We're still not friends.