There are certain actors who I regard as "plug and play". They are very serviceable. They give a lovely, dependable performance. But I'm never surprised by their choices and I never find their performances to be terribly personal. It's this kind of performance that feels like it could be duplicated by any number of other performers without adding or subtracting to the final product.
Then there are the performers who seem to mesh who they are with who the character is resulting in a blend that is in keeping with the story but unique to who they are as a human being. These are the performances where we can't imagine anyone else in the role. These are the auditions after which no one else is even in the running.
There is a universal component to our emotional experiences which allows us to relate to what happens to a character. But it's the specificity of that experience which helps us really believe that the character is having this experience. Love is an experience we all know, but we each experience love specifically. We fall in love with the way someone smells, or the funny way they laugh, or the way they pronounce the word "tortilla". We hate specifically, becoming riled by the way our nemesis breathes, the way they shuffle their feet when they walk, their choice of syntax. Personality is a short cut to specificity. Integrating some of our own weird little quirks into the mix with the character we develop (as long as they don't contradict the givens of the script) helps flesh out everything that the playwright couldn't squeeze on the page. This mesh between character-as-written and actor is what creates a layered, three-dimensional performance.
A lack of personalization can obviously happen in any piece, but I think some works are more prone to it than others. When there's a preconceived notion of how the piece should be done - based on previous productions, or even just general concepts relating to the time period of the piece - it can become a shortcut to playing a generalization rather than forming our own vision of the piece. Classical work often suffers from this. People form a notion of how classical work should sound or move without first answering for themselves how they would specifically respond to these circumstances. Similarly when something is well known you can fall into unconsciously repeating the choices of previous productions, instead of forging the path for ourselves. For example, with something like The Last Five Years, which for a decade had only one recorded version, it's easy slip into imitating Sherie Rene Scott or Norbert Leo Butz because the interpretation that they arrived at (by really personalizing their roles) is so rich.
The other place where I think it can be difficult to find personalization is in broad comedy. In things like farce and slapstick where the physical comedy is such a key component, the specificity of that physical routine becomes choreography. It's not dance choreography per se, but each movement has distinct requirements of which body parts should be where and a tempo at which it unfolds. The trick is to get the choreography to feel honest and true to the characters. And while this is specific and detailed work requiring impressive technical skill, I don't think it stems from who the actor is as a person. Assuming the show has been well choreographed by both the director and the actor and goes on to be well executed by the actors, the physicality does the work. Which is fascinating given that verbal based comedy can be almost entirely tied to personality, where jokes that kill for one comedian fall totally flat for someone else. Broad comedy is one of the few types of theater where we're not looking for the characters to be three dimensional where as stand-up demands a more personal product.
The range of roles an actor can be called on to play are almost infinite. When we look at the types of performances for which actors get Tony nominations (in theory, reflecting the performances the theater industry considers impressive), they tend to fall into two categories: roles that are technically impressive (e.g. the actor plays a zillion characters) or emotionally impressive (e.g. the actor develops a vision of the role that is rich, layered and unique). Certain roles allow for more personality than others. Certain roles demand more technical skill than others. Both of these are essential. To the extent that it's possible, I want as much personality as possible.
Thoughts? Questions? Comments? Post them below. The more, the merrier.