French Knot stitch can be used as a filler stitch or can be used just to fill in small circles.
Pull the thread up through the fabric
Before putting the needle back through the fabric, wrap the thread from the eye of the needle around the point three times, then push the needle through the fabric close to where you just came up. It helps if you pull the thread tight with your other hand.
Pull the thread through nice and tight, the three loops you wrapped around the thread should tighten to form a french knot!
You can then repeat this stitch irregularly across the fabric to create a speckled effect, and even change the size of each knot by varying the amount of times you wrap the thread around the needle.