Cardboard "cookies" can help you produce some amazing lighting effects, like the photo I shared back in 2008:
A Light and a Cookie
Just as often you need large pieces of cardboard to block unwanted light. Back in my college days we used a box to block a window to make it look like night in the room:
Blocking Light from a Window
So I was a little surprised to see a shot from a 2013 film completely ruined by unwanted light. It took me a moment to figure out why the shot felt so odd. It was raining and the image was "off" somehow. What was it?
I've lived in Colorado long enough to have experienced sunshowers, but this wasn't one. The main character kept moving in an out of the shadow, causing the distracting harsh light to be all the more distracting. They could have "sold" the shot by simply getting a large piece of cardboard to cast a shadow where he was standing.
Or have him take four steps back into the shadows.
Remember: Simply spraying your actors with water does not "sell" rain. Watch for the little details and fix them. Sometimes it's as easy as getting a piece of cardboard.
Your Media Production Mentor