We investigate narrative agency in hu-man–LLM creative co-writing, asking whodrives story development in turn-based collabo-ration. Using a new corpus of human–LLM co-written stories, we apply sentiment and seman-tic modeling to quantify affective alignmentand semantic novelty in turn-taking, and direc-tional measures to assess which agent shapesnarrative progression. Our results show asym-metric influence: human turns introduce greatersemantic novelty and are more likely to shapesubsequent developments, whereas LLM con-tributions predominantly elaborate on human-introduced elements. At the sentiment level,alignment is also asymmetric, but more bidirec-tional: LLMs exhibit stronger turn-level emo-tional adaptation than humans, but both agentstrack each other’s emotional valence and LLMsshow an independent tendency to more pos-itive emotional baselines. These findings in-dicate a complementary division of labor inhuman–LLM co-writing, where humans drivenarrative innovation and direction, while LLMsact as adaptive amplifiers that sustain coherenceand elaborate emerging narratives.

Human–LLM co-writing is increasingly used for open-ended text generation, but much prior work focuses on final outputs rather than the interactional dynamics through which stories are produced. We study turn-based collaborative storytelling across three matched conditions: Human–Human (HH), Human–AI (HA), and AI–AI (AA). Using a shared storytelling paradigm, we measure how agents align, introduce novel material, and influence narrative development through turn-level measures of valence adaptation, semantic novelty, transience, and resonance. Our results show that HA co-writing is not intermediate between HH and AA collaboration. Instead, it displays a distinctive asymmetry where humans tend to introduce more novel and persistent narrative material, while LLMs tend to elaborate and stabilize the existing context. These findings suggest that, in this setting, LLMs function less as human co-authors and more as adaptive narrative amplifiers that reshape how agency is distributed in collaborative writing.