A few days ago, I did a exterior car scene lit with only the sun. It was a nice warm, bright winter day, we couldn't ask for anything more. (Except for maybe a HMI haha) But for a low budget short that was not an option so I brought my reflector boards for control knowing we'd need them. Once we started blocking the actor we found the sun light was being blocked by the car's roof so we had to take out our reflectors to bounce light into the cab and key him in. We used the silver side to keep the outside color temperature the same. The gold side produced a weird warm tone that didn't match the outside. I found reflecting the suns light onto the board produced a more white light and was quite harsh from close distance. You defiantly have to experiment with distance from reflector in relation to the subject. Keep the reflector still so the light doesn't move on the actors face.
Here is a still from the shoot.
Eileen Ryan is right, with no sun, no reflection, with no reflection, no light, and with no light, you're stuck. We ran into this problem that day. Crew call was around 7AM. We set up our jib, monitor, audio, etc... by the time everyone was ready it was about 10. Production ran smoothly until about 2PM, when the clouds rolled in, and the sun faded away. We had no light source and were still filming car interiors. We turned off the ND filter, opened the iris, but the picture was still dark. It obviously didn't look the same. With no other option, we stopped filming.
What I would have done different:
There was a bit of light leaking in and spilling onto the passenger seat. I would have placed a white board on the seat aimed at the actor in the drivers seat to fill him in. Also, a small battery powered LED light would have worked in the cab for a more reliable light source. A 6X6 silk on C Stands would be nice to block off any hard light bouncing off snow creating a blown out image. Next time, I'm going to try getting the hardest beam I can off the reflector and bounce it onto a white reflector board placed outside the windshield, hoping this will spread the lighting a little more.
A lot of what you can do depends on the equipment you have available. When you are limited, you really have to think about your setup in advance. Scout out the location, look up weather from a few sources, take a few stills so you can map things out, get people who will not mind holding up a reflector for a number of hours.