Some people rarely speak unless you talk to them first.
Literal
Talk-to [if not-なければ], rarely talk [object-を] not-do people [also-も] exist.
話しかける ('to address, initiate conversation') versus 話をする ('to converse'). ~なければ is a negative conditional: 'if you don't.' めったに~ない ('rarely, seldom') is a set pattern — the adverb always pairs with a negative verb.