In the process of designing and engineering theatrical sound, I've had to do several clever (and far many more patched-together) things to help advance the needs of the show. Below is a list of a few of what I think are more amusing tricks involving actors and microphones.
Telephone Voice ("Guys and Dolls") -- for a real-time telephone conversation, the person on the other end was played by an actor backstage on a microphone. Between him and the speakers was a guitar pedal; a Boss distortion box that provides the compression and crunch of an over-driven guitar amp (the typical sound of electric guitars). The box also has a simple EQ that allowed the telephone voice to be rolled off hard on both sides, mimicking the limited frequency range of POTS (Plain Old Telephone System). With that box, I can dial the telephone effect all the way from "slightly tinny" to "heavily distorted." Although I have done plenty of pre-recorded lines, I prefer when possible to have a live actor that can react to the living timing of the action on stage.
Peter Pan plays Captain Hook ("Peter Pan") -- to be properly convincing, we pre-recorded the lines where Peter was disguising his voice (magically) to sound like Captain Hook and fool Hook's crew. The actress merely mouthed the words. To be really convincing, though, we first had our Peter Pan in the recording session playing Peter imitating Captain Hook, so the actor playing Captain Hook could perform the recordings and imitate the way she would play Peter pretending to be Hook as played by him..... This was the director's idea, by the way, and only underlines my deep preference to having the director on hand for any voice-over session. It takes a director to get a good vocal performance from the talent, and having the director of the actual show there allows them to tailor the recording closer to their needs for the show.
The Ghost Crew ("Rosencranz and Gildenstern") -- the lights go down, and in the blackout transition into the next scene voices are heard calling out orders, as if the stage is the deck of a ship and the crew is spaced around it. And, actually, they were. When I did the recording session, I demanded the actual stage. I set myself up with a stereo mic in the first row of the audience. When we played back the session through the house speakers, the room tone of the original recording session added to the stereo field placement of the performers to make a startlingly realistic re-creation of the original actors. When the sound cue played in that black-out, it really sounded like the actors were actually on stage. (I did a similar blackout cue which was done in-studio; I recorded adlibs from the actors, and did an impromptu foley session in the rehearsal hall -- wearing high heels on my hands, even! -- then edited it all together to produce a short script of actors hissing at each other, running across the stage, slamming doors, etc., in a brief scene of confusion. The result did not sound like it was actually happening on the stage in the dark, but it did sound like it had been recorded all in one shot SOMEWHERE.)
The Ghost of Jacob Marley ("A Christmas Carol") -- to give an extra-ghostly quality to his moans, in addition to a reverse-reverb effect, he had a delay unit on bypass. By hitting a footpedal the operator could "capture" the cry at the end of one of his lines and make it echo until the footpedal was released again. It made it possible to add long echoing tails to some of Marley's cries, without making the rest of his speech unintelligible. (Reverse-reverb algorithm was also one of my tricks for the monster in a different show -- on top of a chorus effect set to double his voice in a lower register and flange it a little. The intent was a slightly cheesy monster).
Yopp! ("Seussical") -- to get the effect of the yell that "reverberated across the universe" the actor was pre-recorded, and the sound programmed to pan around all the available speakers (both front of house and in the rear of the audience). The sound effect is triggered moments after the live actor yells, providing an effect of multiple reverberations from all directions. Some of our multiple cast gave consistent enough performances that the effect was transparent. Others, however, managed to record their "Yopps" a half or whole-step down from how they actually performed it, breaking the illusion somewhat.
Walkie-Talkie ("Rumors") -- to simulate an on-stage police officer getting called on the radio, we pre-recorded a voice and processed it into unintelligibility. Then we sent it to the external mic jack of a walkie-talkie in the booth. All the operator had to do was push-to-talk, and then play the cue, and the walkie-talkie worn by the actor crackled as if getting a call from Dispatch. It actually took a bit of experimentation to get a voice that sounded like it was telling the actor something, but from which few recognizable words could be extracted. The first thing we tried was improvising gibberish. None of our voice actors proved capable of doing this without it sounding like made-up gibberish. We next tried scripting gibberish. That also failed; the voice actors stumbled over the made-up words and their vocal cadences became unconvincing. Finally I wrote out the lines needed, abstracting from the script a telegraphic, terse, but still idiomatic set of English phrases. Then I started processing. Heavy EQ. Compression. Distortion algorithms (the same trick as the telephone voice above). But what really did the trick, and made the cues short enough to work within the timing of the on-stage dialog, was using Stienberg's "Time Bandit" to time-shift the lines to about 1/3 their original length. The end result was a comically distorted walkie-talkie voice in which you could still, barely, make out the words "Shots heard" and "221 Park Place."