Either way, I would make eye contact and smile back. That's just my natural reaction to people who make eye contact with me, lol. I might also say hello. But if it was just in passing, I probably wouldn't stop to actually start a conversation. There are situations where I might though, like if it took place where we were both going into the same place or something, like a coffee shop or a store.
Whether or not you were attractive, it'd be flattering. I probably wouldn't smile back because by the time I realize you were smiling at me I'd be too shy/surprised to do anything and you'll have passed.
I'd have the same reaction for both. I'd give them a quick smile and divert my eyes.
no, I'm not super altruistic. if I were interested, I'd probably kick myself about it later about both. more so for the attractive guy, but both because of how shy and nervous I get. I get kinda panicky when I have attention on me.