Abstract: LLMs are increasingly used across research workflows, including research ideation, qualitative analysis, and participant simulation. Within work on participant simulation, prior studies have largely evaluated simulated participants by how realistic or human-like their responses appear, leaving less known about how researchers use and critique these materials when revising interview protocols before fieldwork. We present Rehearsim, an inspectable research probe that treats LLM-simulated participants as materials for methodological rehearsal rather than substitutes for human participants. Rehearsim combines literature-grounded participant construction, interactive interview rehearsal, and traceable thematic review while keeping protocol decisions under researcher control. In a study with 21 qualitative researchers, participants used simulated materials to surface assumptions and gaps, reconsider participant populations, revise target populations, and develop follow-up probes. They also identified risks, including the concealment of weak questions, dismissal of unexpected synthetic details, and interpretive anchoring from generated themes. We derive an inspectable-rehearsal framework emphasizing traceability, contestability, researcher control, and epistemic boundaries.