Jack O'Lantern stands guard over the Witch's pumpkin patch.  He is a seven foot tall scarecrow whose pumpkin head flickers as his voice echoes across the graveyard. Click Jack's picture to see and hear him on Halloween.  Jack's general construction was pretty simple.  He began life as jasmin vines.
I used the long vines to create Jack's arms.  I cut the vines into lengths and then bound them together in the center and at what would be the wrists.  I really liked having the long vines creep down very far from the wrists, some even touching the ground.  I also took some of the vines and used them as 'ribs' between a sternum and a spine, both of which were cut from tree branches.
The rib cage met up with two bundles of long reeds that I used for legs.  I snapped them so the bent at the foot and at the knee.  For the pumpkin head, I used a foam number that I carved and then painted for dramatic effect.  I also twisted a piece of aluminum foil and glued it to the stem of the pumpkin.  After some paint, it looked like the remnant of a pumpkin vine.
I then draped the scarecrow body in cheesecloth, both natural and black, for a spooky coat that fowed in the wind.  I also wrapped his arm in vinyl screen that hung nicely and was transparent.  It gave him a cool bat-like appearance.  I added some crow and the look was pretty much complete.  Now for the fun part, getting him to talk.
I ended up using a Scary Terry servo driver board to like Jack up because I happened to have one lying around.  You can use any type of light organ for the effect and there are several great tutorials out there.  Basically, I hooked up an audio source to a board that would make a light flash whenever sound was present.  By using a dialogue track and a light inside of the pumpkin, Jack's head would light up every time the voice spoke.  I ran into similar problems with Jack as I did with Harold Noskin.  I wanted a processed audio track for Jack's voice, but all the echo and reverb would make the lights not flash in synch with the dialogue.  Just as with Harold, I ran one channel to the driver and a separate one to the speaker system.  As there was no servo to drive, I didn't have to offset the driver audio track because the light worked at... well, the speed of light.  I will talk a little about the type of audio processing I did to get Jack's voice.
I wanted to give Jack a creepy booming voice.  What I had was my my nasal drawl.  So I started with a raw audio track of my voice affecting the most sinister tone I could.  Click the link the the right to hear it.
Here is the processed audio.  I used several different effects for the voice.  First I shifted the tone down a full octave.  Then I added another that was dropped down only about sixth.  This made the voice sound dissonent and creepy.  Next, I used a very creepy reverb trick.
First start with your raw audio.
Raw File

Next, reverse the audio.
Reversed File

Then add reverb to the reversed audio.
Reversed File with Reverb

Reverse the file again.
File with Front Loading Reverb

Finally, add a bit of reverb to the new file.
Completed File
You hear this ghostly effect a great deal in movies.  It is really simple, but really effective, almost like the spirit is conjuring the energy from the air to make the sound.  The example above is exaggerated to demonstrate the effect, but it works especially well with whispers.  I use the Sony Sound Forge to create these effects.  Despite the effect being great, if you use this audio track to drive a servo or a light organ, it will start lighting up way too early.  I would use a separate clean track of the raw audio to drive the servo or light.  I used a similar process for my servo driven skeleton, Harold Noskin.

As for Jack, I simply ran a low voltage light bulb which I placed in his pumpkin head.  When Jack spoke, his head lit up.  I also added a constant flicker LED inside the head.  This gave the effect of a Jack O'Lantern that was constantly lit and flickering, but increased in intensity when he spoke.  Overall, a pretty cool effect.