I try to sustain the attitude so it's like a lamp: the light shines, but only shines on when something comes into range, and that is a function of the light already permeating the area and not a change in the way the light permeates.
So with the brahmaviharas, it's almost as if each sense sphere is already primed, such that - increasingly as the practice continues - any beings which appear to this or that sense are already appearing with the brahamviharas as background... as though the sense spheres have brahmavihara wallpaper, say, or a brahmavihara fireplace, or brahmavihara incense, or brahmavihara muzak... it just pervades, and equanimously gloms onto whatever beings there are with respect to that sense.
"And how is it, bhikkhus, that by protecting oneself one protects others? By the pursuit, development, and cultivation of the four establishments of mindfulness. It is in such a way that by protecting oneself one protects others.- Sedaka Sutta [SN 47.19]
"And how is it, bhikkhus, that by protecting others one protects oneself? By patience, harmlessness, goodwill, and sympathy. It is in such a way that by protecting others one protects oneself.