I'm not sure I understand the question, do you have any photos to explain? When adding mouth details I needle repeatedly across the line where the lips part to create an indentation and then more randomly above and below th lips so the chin and area below the nose recede a little, creating the appearance of fuller lips.
I like to twist a small wisp of felt into a straight line and then felt it on gently using my needle. If you poke too hard it will create an indentation. If the piece is not too tiny and delicate, you can also go over it with a 3 pronged felting pen which seems to poke more evenly than a single needle.
For a bit thicker lines I roll the wool between my hands first (fast motion with friction, lightly felts it). For thin lines I roll between thumb and finger. I use a single needle to felt onto the piece. Like the idea of the using a three pong for overall felting afterward.