Сказать и говорить
«Сказать» emphasizes the completeness of the process, it implies the result, while «говорить» focuses on the process itself and emphasizes its duration rather than the fact that the process has reached its limit as in «сказать».
«Сказать» means «to express one thing, one thought» (and it may also be used in the meaning “to pronounce”), ...
"Which of these should I use if I'm talking about a coworker of mine?"
This part of the original question was left unanswered.
A female coworker is called "сотрудница", or you can say "девушка с моей работы" for a young lady, and "женщина с моей работы" for an older one.
I would say that сказать is content oriented, it is used when you follow up about what actually was told. It got same root as сказка - tale.
говорить is indeed analog of talking or speaking to if object of action is mentioned, or saying in case of subject.
поговорить is related to verb говорить with perfective aspect of action. The act talking to someone ...