While I don't know the exact method they used on the show, the most practical way of recreating the Alphabet Wall is with RGB smart pixels. This step is just a broad overview of what I used to get the lights going, but I think the links provided will help you more than me rehashing what they already convey very well.

Pixels look like a string of Christmas lights, but each bulb has a small computer chip attached to it, and with the proper controller, you can turn each bulb on/off, set the color, make it flash, flicker, and more.

Lights:

The pixels are available from a variety of shops. I bought mine from Amazon as well as http://www.environmentalled.com/. One thing, make sure that your controller and your lights are the same voltage. I used a 12V controller but accidentally bought a 5v string of lights and filled my office with smoke when it burned one of the chips out. Most of my pixels I used were the smaller variety, because they are a lot cheaper, but I did use one strand of the C9 larger lights for the alphabet wall itself.

Controller:

There are several controllers, but I wanted one that met 2 criteria 1) expandable so that it could be used for future Halloween and Christmas displays 2) Easy to program and interface. There are builds that use Arduinos which would be much cheaper, but seemed to require a steeper learning curve. I was starting this project at the beginning of September, so time was not on my side. I ended up buying a Pixlite 16 MKII. It was pricey, but it seemed to do everything that I needed as well as be useful in future projects. It has 16 outputs, and each one can power several strands of lights.

Software:

There are a few software programs out there to control Pixels. Two top contenders are Xlights and Vixen. I started with Xlights because I liked the GUI better and I found a lot of tutorials out there. One great series is here. I had the show built in Xlights, and I felt pretty good about it...but then I decided to add a kink to the plan.

The Kink:

So, to make my life a nightmare, I really wanted to add a scene from the show where a set of lamps are lighting up in a sequence back and forth. The Pixlite controller is only for smart pixels; the lamps would be running on A/C power. So, to handle this, I had to buy an Arduino Mega and a relay board. I bought a 16 relay board, but they make them smaller. After a lot of trial and error...and more error, I ended up having to swap over to Vixen for the software because I couldn't find a good tutorial on Xlights and Arduino working together. I'm sure there is one out there, but I was more successful with Vixen and the relays. I used this Instructable on programming the Arduino to work with Vixen.

The Outlets & Relays:

This part is dangerous if done wrong, so skip it if you aren't familiar or comfortable with household current. To make the lamps dance back and forth, I needed 5 outlets - independently controllable. Here is a great write up of how to wire up the outlets to the relays.

Once I got everything wired up, I had the Arduino connected to my laptop via USB, and the Pixlite and laptop communicated over my home network. I would link up my Vixen programming, but it is 100% based on the show you want to create, so my show wouldn't do you any good.