Purpose: Patients’ readiness for advance care planning (ACP) is often considered a prerequisite for starting ACP conversations. Healthcare professionals’ uncertainty about patients’ readiness hampers the uptake of ACP in clinical practice. This study aims To determine how patients’ readiness is expressed and develops throughout an ACP conversation. Methods: A qualitative sub-study into the ACTION ACP conversations collected as part of the international Phase III multicenter cluster-randomized clinical trial. A purposeful sample was taken of ACP conversations of patients with advanced lung or colorectal cancer who participated in the ACTION study between May 2015 and December 2018 (n = 15). A content analysis of the ACP conversations was conducted. Results: All patients (n = 15) expressed both signs of not being ready and of being ready. Signs of being ready included anticipating possible future scenarios or demonstrating an understanding of one’s disease. Signs of not being ready included limiting one’s perspective to the here and now or indicating a preference not to talk about an ACP topic. Signs of not being ready occurred more often when future-oriented topics were discussed. Despite showing signs of not being ready, patients were able to continue the conversation when a new topic was introduced. Conclusion: Healthcare professionals should be aware that patients do not have to be ready for all ACP topics to be able to participate in an ACP conversation. They should be sensitive to signs of not being ready and develop the ability to adapt the conversation accordingly.