I'm not great at freehand, but I like to add a touch of drying retarder so the paint flows for longer.
Some of the finer work I've done, like the name tags on the Space Hulk model which are made from super fine lines (double lines and little serifs on the text), I actually used enamel and oil paints. That was because it let me do test strokes over and over again on a bit of painted sprue until the paint was flowing perfectly and it'd keep flowing perfectly for the few minutes it took me to paint the model, and also because oil and enamel paints naturally have good flow, and also because if I screwed up I could keep manipulating the paint on the model or alternatively just wipe it off and start again.
But I'm a weirdo, other skilled painters would probably just do it with acrylics,
lol.