Emerging experimental evidence shows that writing with AI assistance can change both the views people express in writing and the opinions they hold afterwards. Yet, we lack substantive understanding of procedural and behavioral changes in co-writing with AI that underlie the observed opinion-shaping power of AI writing tools. We conducted a mixed-methods study, combining retrospective interviews with 19 participants about their AI co-writing experience with a quantitative analysis tracing engagement with ideas and opinions in 1{,}291 AI co-writing sessions. Our analysis shows that engaging with the AI's suggestions -- reading them and deciding whether to accept them -- becomes a central activity in the writing process, taking away from more traditional processes of ideation and language generation. As writers often do not complete their own ideation before engaging with suggestions, the suggested ideas and opinions seeded directions that writers then elaborated on. At the same time, writers did not notice the AI's influence and felt in full control of their writing, as they -- in principle -- could always edit the final text. We term this shift \textit{Reactive Writing}: an evaluation-first, suggestion-led writing practice that departs substantially from conventional composing in the presence of AI assistance and is highly vulnerable to AI-induced biases and opinion shifts.

翻译：新兴的实验证据表明，借助AI辅助写作不仅会改变人们在写作中表达的观点，还会影响他们后续持有的意见。然而，对于AI写作工具产生观点塑造能力的背后机制——即人机协作写作过程中的程序性与行为性变化——我们仍缺乏实质性的理解。本研究采用混合方法，结合对19名参与者关于其AI协作写作经历的回顾性访谈，以及对1,291次AI协作写作会话中观点互动与意见形成的定量追踪分析。分析表明：与AI建议的互动——阅读并决定是否采纳——成为写作过程的核心活动，这削弱了传统的构思与语言生成过程。由于写作者常在未完成自主构思前就开始处理AI建议，被植入的建议性观点往往成为写作者后续延伸拓展的基础方向。与此同时，写作者并未察觉AI的影响，且自认为完全掌控写作过程，因为他们——原则上——始终保有对最终文本的编辑权。我们将这种转变称为"反应式写作"：一种以评估为先导、建议为主导的写作实践。它在AI辅助环境下显著区别于传统创作模式，且极易受到AI引发的偏见与观点偏移的影响。