The image shown here is shot with a nikon speedlight on a stand with a 83cm white (transparent, shoot thru) umbrella to diffuse it. Im not using a ttl cable, since i use the speedlight in manual mode (i trigger it with a set of pocketwizard units) and meter it manually with my flash meter. However it will work just aswell with your ttl cable. The diffrence is that camera will meter for you.
In the image i have the umbrella camera right, thus the shadows on the left, a little foamcore sheet or similar could easily work a reflector. And would have lighten up the shadows on the left side.
I have a small lastolite reflector for macro work with a flexible arm.(wich i didnt use in the image shown) But anything you can find in the house that reflects light without coloring it would do nicely.
If you dont have a tilt swivel umbrella holder, i highly recommend you to get one.
I have the manfrotto 026
but BH have numerous brands making them, sps and photoflex just to name two.
If you dont have or dont want to use a umbrella, you could just take something like foamcore sheet and bounce the flash of it, since you will be using a ttl cable the camera will do the metering for you, all you have to do is to compensate until you get the exposure you want.
In this thread you can see a the setup (even though, in the first series of images iv used a large sb, instead of a 83cm umbrella)
The first images where shot with the sb800 and with a large sb (isnt very practical to hike with lol)
Non of these images had a fill/reflector, becuse the larger the light source the softer the light. And with a large sb the light wrap around the subject and procuce only very soft shadows.
so a fill/reflector isnt always needed, but for smaller lightsources like a speedlight without any light modifier it will help alot.
Dont know if my rambling made sense in anway,but lets hope it did.