As you can see in the above photographs, fill flash is a great way to brighten dark shadows.
However, if you can, use a reflector instead.
Compare the above photographs.
I dislike the glare on the pumpkin from the flash.
The white reflector brightened the shadow without glare.
The gold reflector was brighter than the white one, and warmed up the color of the shadow.
Shadows on sunny days are cyan (blue/green).
That's because the shadows are illuminated by the cyan-colored sky.
The gold color of the reflector canceled the cyan shadow coloring.
So, if you can, find a cooperative location with an already existing reflector.
This natural reflector could be sunlight reflecting off of a white wall or red brick wall.
Otherwise, use a reflector.
The downside is that you'll need an assistant to hold and position the reflector.
This writer uses circular reflectors that are easily carried when collapsed.
They're made by Photoflex.
Newspapers, foam core, cardboard, and such, can also be used.