Abstract Slow slip events (SSEs) represent an aseismic transient slip mode that repeats quasi‐periodically, near or partially overlapping with the megathrust earthquake zone. However, the process of stress interaction between SSEs and earthquake zones is not clearly understood. Here, we apply an earthquake and SSE sequence model to simulate the interseismic variability of SSE characteristics, and compare to observations pre‐ and post‐2012 Mw 7.6 Nicoya earthquake in the Costa Rica subduction zone, including six SSEs from 2003 to 2012 and four from 2014 to 2019. Our model reproduces the slip distribution of shallow and deep SSEs, and predicts higher peak velocity and larger cumulative slip of SSEs late in the earthquake cycle. This variability arises from the reciprocal stress transfer between SSEs and the earthquake zone. Our study suggests that systematically documented variations of SSE characteristics may reflect the long‐term unlocking process on megathrust faults where slow‐to‐fast slip modes coevolve.