These are some "make the shot work or die" emergency items for your kit:
Camera Remote If you find yourself rigging your camera in an unusual place (in a tree, under a car, and so on), then having a remote to start and stop your camera can be helpful.
Zoom Lens Shooting with prime lenses is the most common choice for most feature films. However, in tight spots with limited time, having a zoom lens that you can snap into a framing and hit Record is priceless.
Variable ND Filter If you are shooting outside in a sunlight/shade mixture, then moving back and forth between different ND filters can really slow down the shoot. If you have a variable ND filter, you can just dial into the proper ND amount and hit Record.
Tripod or Camera Support Stand-ins Tripods are an essential part of production. However, since DSLR cameras are small and light, part of the fun is using them in locations not possible with traditional tripods. Get your hands on a Gorillapod or other support system that allows you to stay small but still have some control as to where you point the lens.
Sandbag Don't want to buy a Gorillapod or other small support gear? You should have a sandbag on set anyway. Throw the sandbag down and place your camera right on top. The sand is somewhat moldable, so you can finesse your camera into position without costing you a dime.
Pocket Level When you use a sandbag or other strange support or mounting system, it is easy to end up not having a level shot. A lot of times you are not able to look directly through the camera if you have the camera rigged in a tight or strange location. A simple pocket level can help make sure you haven't tilted your camera in a way you didn't want.
Car, Skateboard, or Cart Depending on your budget (or if you improvise a last-minute shot), you may not have access to a dolly or Steadicam. No worries-be creative and use a car, skateboard, or cart to help you get the moving shot you need.
Batteries Did we mention batteries? They're not just for the camera; you'll need batteries for any portable lights, remote starters, microphones, and so on. Make a list of all the types of batteries your various gear requires and bring extras.
Color Correction Cards and White Balance Cards Have a folder with a white card and color card on standby. Whenever you set up at a new location, go ahead and shoot each card before your first take. You never know what might help in post.
Tape, Sharpie, and Plastic Zip Ties for Emergency Follow Focus Office supplies might seem out of place for the production crew, but on set you never know what will come in handy. Make a little kit with tape, Sharpie pens, zip ties, twist ties, clamps, paper clips, and Ziploc bags.
Black Ouvetyne Ambient light on your monitors sometimes can't be totally flagged or blocked. If you have some black Duvetyne, then you can throw it over the camera or monitor and see exactly what you are getting in the shot. Focus, color, and exposure are infinitely easier to achieve when you are not fighting ambient light on your monitors or LDC screen on the back of the camera.
Hardware Store Lights and/or Small Adjustable LEO Light Panel Since DSLR cameras are so light sensitive, it never hurts to have some good old-fashioned flashlights, lanterns, and car spotlights in the trunk. You never know when a little splash of light will complete your scene. Battery-powered, portable, and small are key features.
Extension Cords That Work in All Weather Don't use old extension cords from around the house or that are frayed or kinked. Borrow, rent, or buy some high-quality, durable cords that are somewhat weather resistant.
Umbrellas and Plastic Bags News flash-meteorologists are not always right. Bring extra umbrellas for both the equipment and the crew. Also, don't just bring the small pocket umbrellas. If you have your camera on a tripod with a long lens, a tiny umbrella is of little use. Have at least one large umbrella for each camera you have on set. Size matters. Also, have a supply of garbage bags and plastic bags you can drape over set pieces and equipment.
Fold-up Reflectors Sometimes all you need is a little bounce. Have a large foam core bounce card is sometimes impractical or unnecessary. Get a set of fold-out reflectors (white, silver, and gold), and in seconds you can have a light, portable, and flexible reflector to help you get the perfect shot.
Black Wrap and Clamps When shooting with cameras that are so light sensitive, sometimes it is more about subtracting light than adding it. Using some black wrap around a light can help you take away some light or light leakage from your scene. It's cheap and easy to use. Make sure they are part of your pre-production checklist.
White-and-Black Poster Board Just as fold-out reflectors have their place, so do large white-and-black foam core boards. These can be added to help set up a large soft bounce source or create a barrier to keep out light.
Dimmers and low-Wattage Bulb You would be surprised how bright a table lamp is if it has even a 60w bulb in it. If it has a lOOw bulb, it is even worse. It is nice to have a box of low-wattage bulbs that you can change out in lamps, ceiling lights, and chan deliers to help you get the correct lighting for your scene. If you don't have low-watt bulbs or if your low-watt bulbs are still too bright, then having a cheap $10 dimmer from Home Depot will save you. Have a couple on hand, and you can put all the incidental lights on a dimmer and in minutes have all the lights balanced and matched.
Extenders and Extension Tubes If you have only a few lenses, then don't forget to get an extender and extension tubes. An extender can take your 100mm and turn it into a 200mm or 400mm lens and help you get the long shot you didn't think you could get. Conversely, if you don't have a macro lens and need to get a close-up of someone writing a letter or the cursor on the computer screen, an extension tube will change your focal plane and make any lens you have available able to focus at a much shorter distance.
Print It Out Print a copy for yourself and a backup copy. Of what? Everything: the script, schedule, crew contact sheets, actor releases, and so on. With everyone having a laptop or portable device, sometimes we forget to have a printed copy you can hand to someone. If you run out of batteries and don't have a printed copy, then you are out of luck. Again, it's just another backup that you will be happy you have at least once during your shoot.
Coffee and Bribe Money Never underestimate the power of a cup of coffee or a few extra dollars in cash. You will without a doubt be thanking crew members and people from whom you secured locations, props, or equipment more than you can remember. After a while, you might need a little extra to help you finish a shot or ask someone to stay just a little longer. Always make sure to have fresh, good coffee (at all hours of the day) and an envelope with some cash. Never forget how far $20 to $50 might get you at the last minute and help you finish your day.