At first glance, voice acting may seem less complicated than 'full body acting' for stage or screen. After all, a voice actor doesn't have to worry about the immediate effect of their physical actions or physical appearance on the audience. However, acting is communication in action, and according to research by Albert Mehrabian, Professor Emeritus of Psychology at UCLA, speech only accounts for 45% of communication.
This breaks down as only 7% of communication being words, with tone of voice being 38%.
Voice acting is not simply stage/screen acting with the physical removed. As an experiment, listen to an extract from an unfamiliar movie with your eyes shut. The actors will be using their voices to create character, but in harmony with their physical movements and their surroundings, so it will usually sound like a movie with the picture part missing. Listen to a similar dialogue in a radio play, and the acting should sound 'complete', so that the audience is not distracted by the lack of visuals.
That's not to say that voice actors neglect their physicality. Posture, gestures and facial expressions all affect and change the sound of the voice. A smile will carry over the airwaves, as will tension or energy.
While the limitations of audio can actually be liberating, they can be a challenge. The voice alone has to do the job of all the other senses, so voice actors learn to use it rather like an instrument. The actor is left only with the words, noises (and the rhythm and space within them) with which to express meaning and emotion, but also accent (and related background nationality or ethnicity; age; class; implied physique; speech impediments, etc). Here are some ways in which they may do that:
To get different tone, the actor can use different parts of their body to resonate the sound, like head, nose, chest, etc. to make the voice sound reedier, more nasal, broader and stronger (as it goes downwards).
To find accent, they will usually study the accent from authentic examples (found on Youtube, or on specialist sites such as the fantastic IDEA one. They can practice the shape of the mouth and throat and, eventually, be able to apply those sounds to a script.
Interpretation and Emphasis
Note that Mehrabian's studies showed that tone of voice is responsible for well over a third of communication. Meaning leads to a lot of experimentation and practice. Audio scripts typically have few directions if any, and a sentence like "What are you doing?" has 4 different interpretations to start with, simply by emphasising a different word. Add an emotion, or several and all the takes mount up.
Emotion is surprising - as mentioned previously, a smile does travel extraordinarily well over the airwaves, and if the actor is tired, it's quite frightening how the microphone can pick that up. In Old Time Radio, emotions tended to be almost as intense as in stage performances. Nowadays, the general trend is much more for 'Realism', so listeners' ears are tuned in to hear subtle emotional performances.
Of course, actors can do all of the above in a stage play, as well as a whole lot of physical exercises to find posture, movement and so on, but the listeners' focus on the voice alone and the intimacy and sheer sensitivity of a microphone make accuracy and control essential.
Voice actors also have to take the microphone's sensitivity and quirks into consideration when breathing, making louder noises, positioning themselves (and not accidentally shifting about while speaking). The microphone is an extension of their body and voice, in a way, and so they need to master microphone technique to avoid puffs or clattering about.
For satellite audio drama, actors may be cast around the globe. They record several takes of their lines in home studios and then send them back to the mixer/producer. This may happen in screen acting, too, and actors need to work hard on sounding as involved with their imaginary fellow actors as possible.
The mixer can help in treating the lines to make them sound as if they are recorded in the same size/shape space, but it is the actors who can make it truly believable.
My own experience in mixing satellite audio soon taught me that the actors who sounded most natural in their conversation, when mixed in with another characters' lines, were ones who said the words in the same way: they all delivered any dialogue lines as if they were unfinished.
So instead of the listener almost hearing the deadening crash of the question mark in:
"Lovely to see you! So how are you?" the actor would -sometimes almost imperceptibly- continue. It might just be a short intake of breath; or a tiny, abstract vocal sound; or a very slight elongation of the last word in anticipation of the answer. Or occasionally, they would actually begin the first sound of a new sentence.
When they were then placed in a conversation, those extra bits would ride gently under the other person's response and it would compensate considerably for the conversation being recorded separately.
To do this, the voice actor imagines themselves really having that conversation. Perhaps they improvise with it in their head (and often in takes, as well - which is always an extra blessing for the mixer). I personally found that reading and 'hearing' the other characters' voices responding to the lines helped in getting into that mode.
So all in all, voice acting is a slightly specialised form of acting, with the detail that such an intimate medium brings. The aim for all voice actors is to gain control over a wide range of aspects of their voice, in order to express outwardly the character they are feeling from within. Like all creative processes, there are large elements of craft mixed with imagination and experimentation until they find the voice that fits.