I thinkI you need a swirling motion to create lather -- whipping up lather with a brush is similar to whipping cream, or egg whites, with a whisk. So what happens is that you move something through a surface-active mixture (cream, egg whites, soap solution) with enough speed to create a vacuum in the wake of the moving object that gets filled with air. Because of the fineness of the hairs, the air bubbles introduced in soap lather are extremely small, creating a nice creamy lather. A linear motion, especially a painting motion, where the brush does not really move 'through' the lather would not be effective enough.
So to create the lather you need to swirl, either in a bowl, after which you can just paint it on (which is what I do), or on the face, when face-lathering.
Stated differently, you can only paint if you already have a lather.