As an actor, every time you create a character, every time you get an audition, every time you book a job, something different is asked of you, as no two characters, scenes, or productions are identical. In fact, typically there are vast differences between the things you will be asked to create, the characters you are asked to portray, and the environments in which you will be asked to work.
Some of what you are asked to create (especially in film and television acting) is real, and some of what you are asked to create is conceptual, and that combination is ever-changing.
"The worst thing about scripts is that they're already written." - WCJ
Actors are asked to say words and do things as someone other than themselves as if those words and things are happening for the first time. What's more, in a typical day on set, actors will likely have to do and say those things over and over again, each time making it feel anew.
In most sports and performing arts, a great deal of what you are working with is real, tactile, and happening at the time; acting, on the other hand, is full of abstracts and missing pieces (e.g. falling in love with someone you don’t actually love, hiding from a CGI dinosaur, etc.). The demands are always changing, and much of what an actor is dealing with is not really there or happening.
A great acting career will include a wide array of varied content; the greater your “range,” as an actor, the more likely you are to have an enduring career.
The skill-set that must be developed and owned for each role is different. It may be different in the type of character an actor is playing, it may be different in the situation the character must deal with, or it may be both. In a typical film role or television season, an actor may need to create multiple skill-sets and behaviors (changes that the character experiences), akin to playing many different types of sports and positions, all in one season.
Typically, the only structure that is afforded the actor is the script and the set; both of which are abstract, incomplete creations. And, just as reading a playbook is an ineffectual way to learn to play basketball, we believe the script represents a poor practice tool as it does nothing to activate the actor’s instrument in a way that leads to the creation of human expression and believability. It is what the actor adds and creates within the script that counts.