Dealing with light in unpredictable situations is hard, but if you follow a few principles you may be in luck.
Let There Be Light
Light is one of the most difficult things to control in natural environments. All filmmakers wish for control over this element because it is simultaneously what makes images visible and can wash it out completely. When out on a guerilla mission you do not usually have the time, resources, or access to use artificial light set ups or do many environmental alterations to make the light more palatable. There are a few things to do when you get to a location to try and work out the issues with natural light and then make the image come out superior.
The most important thing to do when you enter a location is to identify the main light source. This could be the sun or a lamp of some sort. If the light source is white to some degree then you should try a white balance on this, unless it is very dark in the location. If it is too yellow of light then you should try and place a white card next to it and white balance on that. If you are outside in the bright sun you can try and white balance on the sun or a cloud near it, or you can try and find where the main object of light is and place the white card for balance there.
You are not going to have much control over the exact movement of your objects, but you can try to coerce them into certain locations. Try to get them to places that have a nice balance of light. This means that it will not be too strong on them but will illuminate them to the appropriate level.
Remember the dramatics that light produces and see if you cannot either make the reflecting light soft or more dramatic with heavy falloff. Middle of the day sun will create this strong shadow almost no matter what, so this may either be the perfect time to shoot or the most absolute moment to avoid. If it is overcast out you can expect for the image to take on a much less dramatic tone.
Try going with some of the neutral density filters for very quick darkening of images. Try these out far ahead of time so you know exactly how the image will look, then when you hit a bright spot you can quickly switch on the filter. This is perfect for unpredicted bright lights or sun shifts. Try to work with the iris as well, but this is harder to move around with spontaneously.
The most important thing to keep in mind is that the light needs to work in your favor, so decide how you want the image to look ahead of time and then work towards that.