This study investigated how task complexity influences second language (L2) writing. Specifically, it focused on the possibility that prompts can create different demands at different stages of L2 writing production and on ways that prompts may affect learners' engagement in the revision of their writing. Fifty-one English as a foreign/second language participants, divided into two groups, wrote a narrative essay. One group was asked to write about a past success (the recalling prompt, high in formulation demand). The other group was asked to write about an imagined future success (the imagining prompt, high in conceptualization demand). Both groups wrote a draft and had unlimited revision opportunities. Writing performance was evaluated in terms of the complexity, accuracy, fluency, and lexis produced in the participants' first draft. Learner engagement was assessed in terms of the time the participants voluntarily chose to spend on revision. The findings indicated that (i) the recalling prompt resulted in more complex but less accurate sentences than the imagining prompt; and (ii) the recalling prompt resulted in more extended engagement in revision than the imagining prompt. The findings suggest linguistic and behavioral influences of prompts on writing. Pedagogical implications are offered.