The word "guy" being used other than with an adjective seems odd to my ears. Is there a rule or explanation as to why that is?
These sentences sound ok with either "guy" or "man" or "boy":
- He's a big guy/man/boy
- That guy/man/boy is looks tired (That being the adjective here)
Whereas these don't. Replacing "guy" with "boy" or "man" seems more natural :
- A guy is coming this way
- One guy is enough to bring the wall down
EDIT: Is it all about familiarity? If you know the man or if at least, it's a specific man, "guy" seems to fit. In the example "A man is coming this way", you presumably don't know the man. In "One man is enough to [do X]", it's not about a specific man. Any man could do X. And more often than not, you'd use some form of an adjective for a known or specific person, which ties back to my feeling about the adjectives..
".. if he'd said stay home, a guy is coming by, I'd have fallen for it.. "seems a little bit along the lines of "a guy is coming by to fix the TV" or "I have a guy to arrange my travels", which in my head, translates to "TV guy" or "Travel guy", and sounds normal. But in the other example,But if a guy is coming out, people gonna take the negative outlook firstis any random guy, and this does indeed sound odd to me. (There is an "are" missing between people and gonna, so I don't know reliable this is) May be it's all my imagination, I don't know.