"If not for his wide, thin lips, perpetually drawn in a single disapproving line, she would have liked him too." [duplicate]

For example, instead of

The guy who is beside me is a jerk

I can say

The guy beside me is a jerk.

It is okay if I don't use the relative pronoun here, either way is correct. But instead of "The guy who sits beside me is a jerk" I cannot say "The guy sits beside me is a jerk." And I cannot say "The people sit beside me are jerks." or "The people sit beside me keep changing." etc.

• What is the rule about this? In what situations do I have a chance to not to use the relative pronoun?

1. The guy who is beside me is a jerk.
2. The guy beside me is a jerk.

The rule we use here is a reduced relative clause (hereafter, RRC).* RRC allows you to delete the relative pronoun (and sometimes the form of BE^†) and transform the remaining elements into a different kind of modifier. #1, for example, is a noun phrase modified by a relative clause; but the reduced form #2 is a noun phrase modified by a prepositional phrase—'beside me'.

3. The guy who sits behind me is a jerk

RRC can also be applied to #3. The relative clause can be transformed into a post-modifying gerund-participle clause.

"The guy sitting behind me is a jerk."

*_{Sometimes analyzed as a whiz-deletion because the deleted words are usually the wh-word + is (whiz).}

^†_{Forms of BE are be, is, are, was, were, and am.}