We present a new approach to investigating student reasoning while writing: real-time capture of the dynamics of the writing process. Key-capture or video software is used to record the entire writing episode, including all pauses, deletions, insertions, and revisions. A succinct shorthand, "S notation," is used to highlight significant moments in the episode that may be indicative of shifts in understanding and can be used in followup interviews for triangulation. The methodology allows one to test the widespread belief that writing is a valuable pedagogical technique, which currently has little directly supportive research. To demonstrate the method, we present a case study of a writing episode. The data reveal an evolution of expression and articulation, discontinuous in both time and space. Distinct shifts in the tone and topic that follow long pauses and revisions are not restricted to the most recently written text. Real-time writing analysis, with its study of the temporal breaks and revision locations, can serve as a complementary tool to more traditional research methods (e.g., speak-aloud interviews) into student reasoning during the writing process.