The sample filler I'm going to demonstrate with is pink wax flower. It is also available in white, purple and orange (in the fall). Other great choices would be Baby's Breath (Gypsophila), limonium, statice and other small headed or massed flowers.
Wax flower has a tough stem with many shoots (laterals) coming off the thicker woody stem. Cut off these laterals and insert quickly between the roses. Don't be afraid to tuck them in deep. The pink is a lovely contrast to the deep red of the roses.
I continue adding in more wax flower.
Here's a view from a little farther back so you can see how the wax begins to "fill in" the gaps.
You can leave longer stems for down the length of the cascade.
Don't fill so full that you take away from that lovely greenery mixture.
The filler flowers is now really beginning to pop out that red contrast.
You'll notice that I kept the filler short and inserted deep, letting it frame the rose heads rather than make the wax flower the same depth.
Here's a closeup of these lovely flowers.
Turn your bouquet around and don't forget to fill in the sides.
The finished bouquet with filler now has a whole new look than before! Remember to allow the red roses be the main focus, while the filler accentuates and compliments that focal flower.