As an actor, my approach to a role varies according to the character I'm playing, the type of script I'm working with, the medium and the creative team. There is no 'one fit' solution. My training at the Royal Central School of Speech and Drama always placed emphasis on the word 'Central'. There was no one way to approach a role, but rather a huge number of techniques that could be drawn upon, a (central) synthesis or convergence of methodologies, depending on both the objective requirements of a part and the subjective element of what the actor responded to on any given day.
Like most Western actor training, the spine is provided by the teachings of Konstantin Stanislavski - a technique that in itself is fluid as it developed over its creator's lifetime. Essentially, you work out the character's NEED TO SPEAK (Why am I speaking? What do I want? Who am I speaking to?) and move from there. In a piece of naturalistic drama, this is often the first approach for the majority of actors - you find the objective (what a character wants) of a given scene and also the super-objective (what the character wants in life/the course of the play), and then investigate the multitude of actions or tactics that might be employed in order to achieve that objective. You might then employ what Stanislavski termed 'the magic IF' in order to bridge the imaginative gap between a character's situation and your own. The actor is not the character and will almost certainly lack any personal experience of the situation the character is facing, so locating a connection between the two is crucial.
But other types of work might invite different approaches. Investigation of a character for a theatrical performance would necessarily involve movement and voice work, and this may in turn inform the character's speed of thought, how they speak, move and even listen. Many actors seek to work purely on an instinctive level and might feel that a cold listing of objectives or actions kills the creative spark, turning the instinctive into the prosaic. They might want to start with the physical, using something like Laban's sequences of movement, as a way of investigating how a character might feel and react. Or if working on a heightened text such as Shakespeare, they may want to focus on the language of what it is they're saying and see how that unique combination of vowels and consonants, metaphors and antitheses etc might effect them on a gut level. All of the above (grossly simplified) plus a whole host of other methodologies (from Method to Meisner) might be more or less effective as tools depending on the individual, the material and the day. Only through practise do you get an increasing sense of what you respond to best on a job-by-job basis, with the caveat that this can change dramatically depending on the demands placed on you.
In voice over, obviously the crucial difference is that you are only heard - never seen. But I believe the same tenants apply. There is a huge variety of approaches available to the VO artist, regardless of the material, and the crucial thing is to explore them CONSCIOUSLY and note their effectiveness in different contexts.
Many VO job's simply ask you to place yourself in the position of the speaker and ask 'What do I want? From who? How can I get it?". From this starting point, you can quickly give yourself a character to play, and also create a real relationship with the person you are talking to - the listener. I'm always surprised by the effectiveness of returning to the basics - imagining a credible relationship between yourself and the listener, identifying what your objective is and then coming up with a set of tactics/transitive verbs to help achieve that objective. The crucial thing this approach encourages is a relationship to the audience. You have to talk TO them, not AT them.
But if you're feeling a little flat, tired, or just unresponsive to such an approach, it's important to have other techniques at hand so you can still deliver quickly - just like that actor working through a half-empty matinee performance on a show they know backwards for for the hundredth time.
Sometimes I find it useful to just think about the energy of the character - their pulse. Everyone has a natural rhythm of delivery that they tend to favour from desperately urgent to virtually catatonic, and it can be useful to click a rhythm when initially reading a text and seeing how this rhythm, this 'heartbeat' naturally settles with your delivery. From there, you can simply try changing the frequency of the clicks to help inform your delivery without having to engage in an intellectual process as to why.
During my Voiceover Kickstart training, one of Guy Michael's first lessons was teaching us about legato (smooth, uninterrupted, sustained) and staccato (shortened, separated) delivery. Everybody sits somewhere on the spectrum between the two extremes, and it's vitally important to identify your own natural tendency before consciously exploring it. I quickly found that I had a natural favouring towards legato, rolling and extending vowel sounds which made my natural delivery automatically suited to soft sell/luxury sales pitches. But for more high-energy jobs or highly technical language requiring clarity, this often became a barrier. Just an awareness of this spectrum gave me something to play with and allowed me to approach a second or third take at a script purely in those terms - automatically providing variety without the need for a wholesale reappraisal of what the script required.
Another approach is: Different Thought, Different Note. Though essentially a very simple idea (changing the tone and pitch of your voice every time the thought in the script changes), an exercise such as this soon reveals what notes you like favouring on a consistent basis. As well as encouraging a greater awareness of vocal variety and the technical practise inherent in that, it also helps clarify vocal tendencies and a sensitivity towards when these can be useful and when they become a barrier to progress.
One's body and its relationship to physical space is a vital tool to credibility and vocal variety. Being able to gesture as freely as you can when speaking will always help your delivery, connecting thought with action. Even when simply listing things, just being able to point towards different spaces in front of you will subtly colour each word and give it more texture. That feeling of space can also be brought inside the body too. It's worth trying the same script where your only focus is making the voice coming from a different part of your body (ie. back of the throat, chest, nose, lips, cheeks) and see how much variety all these approaches can give you for FREE.
But the key lesson is to make it a conscious process. My biggest strides in VO performance have come when I have made deliberate, clear choices and then used those to inform any subsequent takes. Though some of my best work can be the results of instinct, I can always look back on my initial intent as a guide. And believe me, when a client asks you to re-record one sentence in a 800 word script you managed to rattle off perfectly in one take over a month ago, you'll be especially grateful for the note at the top of the script that reminds you how you hit that tone of delivery in the first place!