The through line, sometimes also called the spine, was first suggested by Konstantin Stanislavski as a simplified way for actors to think about characterisation. He believed actors should not only understand what their character was doing, or trying to do, (their objective) in any given unit, but should also strive to understand the through line which linked these objectives together and thus pushed the character forward through the narrative.
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edit:
1950 E. R. HAPGOOD tr. Stanislavski's Building a Character viii. 113 It is only when our feelings reach down into the subtextual stream that the ‘through line of action’ of a play or a part comes into being.
-OED2

assignment for student: Google[Books] "through line"; count theater vs. railroad usages

Last edited by tsuwm; 06/26/2006 4:18 PM.