Vocal processing is one of my favorite challenges. It's always a delicate balancing act to add enough treatment to shine through onscreen without covering up the nuances of the performance. Inventing new ways to treat dialogue is a blast but I also take great pleasure in trying to ape a classic vocal effect.
In an effort to recreate the treatment applied to C-3PO (from Star Wars you guys) my first thoughts were to apply a vocoding effect paired with a reverb based on a small space impulse response. I assumed it was a vocoder making the voice robotic and a reverb placing the voice inside the robot itself. In listening closely and experimenting, it turns out I was half right. I was on the right track with the reverb but the vocoder isn't quite right for 3PO. I ended up experimenting with a flange effect, which hit just right.
Step 1: Apply Flange
My Flange of choice for this instance is MetaFlanger by Waves. What you're ultimately looking for here is a slight chorusing effect. Using MetaFlanger, I was able to apply a light flange with a relatively slow modulation rate, giving a robotic character to the performance and supporting the idea that something is moving behind the voice. You can use any flanger you have available, however I would stress keeping this effect relatively light (I ended up with 50% mix on mine). Less is more here.
Step 2: Apply Reverb
I always keep on hand a reverb that is based on small space impulse response. This tends to come in handy a lot when processing vocals for a mix. Do you have a character with their head in a fishbowl? How about one wearing a medieval helmet? This is where a reverb like Avid Space (or your own personal favorite) can really shine. My first inclination when choosing an impulse response was to go metal. C-3PO is a robot after all. However, the metal response made everything too tinny. Perhaps it was the combination with the already in line Flange effect. Regardless, I needed another approach. I ended up using a Djembe impulse response, which is relatively small and made of wood. I realize wood seems like an odd choice, but it seemed right to me ears so I was sold!
Is this how the folks at Skywalker came up with the sound for C-3PO over 38 years ago? Definitely not. They probably piped it through some crazy outboard gear and recorded it while playing back into a tin can. Short of having the time and gear for that fun stuff, this is a great way I've found to approximate it in your studio.
At this point, it would be helpful to provide you with an example of how this vocal treatment sounds. So, in accordance with copyright law (and counter to my own self interest) I provide you with the following clip; a recording of me doing a terrible C-3PO impression. My sincerest apologies to all die hard Star Wars fans, actual British folks and my co-workers (who were subjected to this for the last twenty minutes through the walls here at Boom Box Post). Enjoy!