(apologies for the blunt wording, pretty tired, very long day on set today and we're not getting our proper turn around.)
Make sure that you DP/OP and your ACs are familiar with the camera. Know what build version you are using, and if it is an older build (like 20 for example), see if your tech's can update the firmware before you shoot.
Overheating is not as much of an issue as it used to be, but it is still an issue. Make sure that you have at least 2 drives (3 is better), at least 4 batteries and 2 chargers. If you are using CF cards, the more the merrier. You'll get about 8 minutes of roll per card. (16GB).
<caps lock for emphasis> IF YOU ARE USING CF CARDS, BE ECONOMICAL WITH CALLING ROLL AND CUT. NOTHING IS MORE ANNOYING THAN HEARING THE CALL FOR ROLL CAMERA THEN WATCHING THE % AVAILABLE TICK AWAY WHILE KEYS HAVE A CONVERSATION FOR THE NEXT 3 OR 4 MINUTES. <end annoying caps lock>. This gets more annoying when you yell at our ACs because of a roll out after pulling one of these "roll for 4 minutes while people figure out what they are doing" tricks.
Lessie, make sure that your crew has the manual that is for the correct build version of the camera.
Never aim a laser at the sensor, and never aim it at the sun either - both things can and will damage the sensor.
x2 on getting your shooter a good 1st AC, 2nd AC, and DIT. Forget "if you have the money." FIND the money. RED One is a touchy, finiky B^&ch, and she will shaft you at every opportunity.
Get used to reboot times when you change batteries, and hopefully those awesome ACs you find will know to fit battery changes and magazine changes into times when camera can be down for a few minutes.
PULL FOCUS!!!!!! Cannot emphasize that enough. If you plan to ever project this footage, don't just trust eye focus on the small LCDs. Even the big monitor can steer you astray in certain circumstances.
And take all the above advice regarding post workflow.
Don't make your DP/operator hand hold the thing for 12 hours. It's murderous.
Do NOT try to use a fully built RED on support gear that cannot hold the weight. This goes for tripod heads, jib arms, car mounts, anything. Yes, the *body* only weighs 17lbs, but when you throw on handles, top and bottom rods, battery cage, battery, drive, top handle, glass, matte box, follow focus, and any audio kit for jam sync or scratch tracks it can easily top 50 lbs. Support gear not designed for the weight WILL fail.
Your DP should occasionally be viewing the raw sensor image to ensure that exposure is in bounds regardless of your look settings.
Remember that when you plug in the EVF and RED lcd, you will loose menu display at video village. Scripty might wonder what happened to the mag/clip display, and there will be no indication that camera is rolling on the monitor at village.
Backups, Backups, Backups. Do it on set. Don't wait till later to backup your data. Make copies.
If the FFD (backfocus) is not set by your rental house, make sure that someone in your camera crew knows how to A: check it, and B: fix it if it is off.
If you are using the base RED battery cage, have your crew check it. A wonky one can really ruin your day by randomly shutting down the camera when it looses power.
I could probably think of more, but I'm pretty out of it at this point.
RE: Scarlett. At his point that thing is vaporware, and there are rumors of it going extinct before it is even on the market.
Spinner, the last info on Scarlett is that there is a fixed lens version and that pricing for the other versions are for body only as with the One. Again though, vaporware that may never come to market.
Edit: Certain frame rates only work at 2K, and even then only on a drive, not a CF card. There is a chart in the manual (again, use the manual that matches your build) which details what frame rates you can shoot at what resolution and on which media. You *can* shoot higher frame rates at 3K, but I have talked to a few DIT and Editor types who have had problems mixing 3K and 4K footage, and thus prefer to overcrank at 2K if the frame rate goes over a certain speed. Remember that doing so is essentially cropping the sensor, thus giving you a field of view that is equivilant to 2x the focal length of your lens. IE: a 25mm lens at 2K gives a similar field of view as a 50mm at 4K. This is not really a "zoom" or "punch in" effect, it is an effect similar to crop factor on SLR cameras.