*Update*
We were working on the "Crystal Cave" sequence, and we came across a problem: the lights we used did not look right, the cave looked fake, and the crystals were very hard if not impossible to see. We made the crystals glow in post-production, but the cave still looked fake due to the fact that the light in the rest of the scene was very drab and normal. We considered using Christmas lights, but they are very clunky and a big pain to work with. Because of this, we decided to come up with our own effect to get the job done. After much experimentation, we found that using After Effects' 3D lights was a very effective solution, and with a little tweaking, we came up with this:
Before:
After:
As you can see, it is a very subtle effect, but it works (in our opinion) nearly if not just as well as filming with Christmas lights. Also, there is the plus side of having an enormous amount of control with the colors, position, and brightness of the lights, which you don't really have with Christmas lights. Also, please note that the images above are TESTS. They are not actual footage from the film, and the effects you see are not finalized. However, that does not mean we don't want critique. On the contrary, please let us know what you think needs doing in this shot, and we will take whatever you say into account. Also note that the jewels are DEFINITELY NOT a finished effect, and we are aware that they look very fake.