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.
"There is, headman, dhammasamādhi. If you were to obtain cittasamādhi in that, you might abandon this state of perplexity. And what, headman, is dhammasamādhi?- SN 42.13 - Pāṭaliya
[kammapatha & brahmavihara, & a method of arousing gladness]"
"Others will misapprehend according to their individual views, hold on to them tenaciously and not easily discard them; we shall not misapprehend according to individual views nor hold on to them tenaciously, but shall discard them with ease — thus effacement can be done."- MN 8 - Sallekha Sutta