If Masterchief and or the Arbiter walked up to me, I would probably invite them around to my place to see if I could actually beat them in a few rounds of ACTUAL Halo 3 multiplayer.
If I could beat the chief at his own game I think I'd feel pretty fucking good.
If Alix Vance from Half Life 2 came up to me I think I would nervously ask for her phone number then run away.
If Solid Snake came up to me I would politley greet him but try to avoid geting into a conversation with him because once you get him talking you can't shut him up.
If Shepard, Wrex, Ash, Liara, Tali, Garrus and Kaidan from Mass effect showed up at my door step I would so welcome them in and have a keg party and try my luck with one of the ladies. Given that in my version of Mass Effect, Ashley is the lover of Shepard, I guess I'd try my luck with Liara.
I'd also buy Wrex a couple of beers.
If Raikou from Otogi walked up to me, before I could even speak he'd smack me with his sword and send me flying through seven walls. I would get up, dust myself off and say something along the lines of:
"Awesome."
If my character from Saint's Row 2 showed up, I would play classical music for his sake and try not to make eye contact...
If Either of the Carmines from Gears of War showed up I would hang out for a while, then do everything in my power to get them be more cautious for Christ's sake.
And if my Argonian character from Oblivion arrived at my house, he and I would just have a nice chat while he dicks around on my rooftop.
If Ivy from Soul Calibur came up to me, I get the feeling we would then have incredibally rough, dirty, sweaty, naked, angry almost animalistic sex and when she left I'd be nursing my many sex injuries including bruises and broken bones. I wouldn't be mad at Ivy though because by this point I'm used to this sort of thing.
Finally if Sonic the Hedgehog showed up at my doorstep I would tearfully (but unflinchingly) blow his brains out with a shotgun and call it a mercy kill.