when/how did plot events develop?

in my reading of Anne Carson's, Autobiography of Red, I kept returning to this question: when/how did plot events develop?

what do i mean? well, sometimes in my own writing i will have a particular plan in mind for a piece of work. i'll have the major plot events mapped out (to some extent), and i feel rather in control of the work. other times, i like to begin with just the beginning and see where the initial idea takes me. i feel less in control of a piece of work when this is the case, but inot necessarily in a negative way. both patterns have worked for me @ one time or another; they are just differnent systems, i suppose.

while reading Carson's book, i kept wondering when and how she decideed to make particular shifts in the plot or to develop new events? a huge example of this begins on page 125, XXXVII. EYEWITNESS. did Carson plan all of the preceding plot events after selecting this particular group of events? or did this event-- Geryon finding out about his origin-- stem from the rest of the plot events?

(hope this makes sense. its not really a question, more so a comment, i realize this. but did anyone else have questions similar to mine? in your own writing to you have a style or a pattern for developing plot ideas?)