Background: Gatekeeper training (GKT) is a common strategy in suicide prevention that educates informal helpers (e.g., friends) about warning signs and risk factors for suicide as well as strategies to support distressed peers. "Sources of Strength (Sources)" is an effective intervention model that combines GKT with school-wide prevention activities and relies on trained gatekeepers--"peer leaders"--to diffuse intervention elements into their social networks to reduce peer distress and promote wellness. Objective: The present study identified internal characteristics of peer leaders that make them successful gatekeepers for "Sources" and explored how "Sources" skills are successfully transmitted into peer leader networks. Method: Using qualitative interview data from two cohorts of peer leaders, we investigated diffusion and intervention efforts during high school and in post-high school environments. Results: Results support the use of the Theory of Planned Behavior (TPB) in understanding behavioral precursors that encourage effective gatekeeping (GK) behavior. Findings also suggest that "Sources" diffusion was robust in high school but declined post-high school. However, there were important inherent characteristics that appeared to spur greater uptake of "Sources'" GKT, including optimism, empathy, kindness, and extraversion. Conclusions: TPB offers well-defined constructs that can be targeted to promote GK behavior in the context of "Sources" or similar suicide prevention programming. Specific internal traits may reinforce gatekeepers' behavioral intentions, which then may support greater engagement in GK behavior. However, declines in GK behavior may occur over time, which suggests post-high school booster activities may be important to offset such declines.