(Warning: This is not an ideal solution when you consider that a tub of filler is often less than $10 🙂 )
Use Toothpaste to Fill Holes. Toothpaste is a great alternative to filler if you have a hole in your wall smaller than 6mm. Squeeze the paste into the hole and wipe off the excess with a putty knife or playing card.
NOTE: You can even paint over toothpaste when it’s dry.