Minecraft wiki entry for Combat, associated text: If there is 3 blocks or less between the player and the creeper, and it has a line-of-sight to the player (i.e. the player and the creeper can see each other's faces), they will start its 1.5 second countdown before they explode. Hitting it and moving back or sprint-hitting it will move it far enough away that the countdown stops. Creepers running away from cats that have started their countdown can still explode if the player is close enough.