This question has two considerations and I think the first one changes depending upon where an actor is in his/her career.
In the beginning, it’s a good idea to audition for any part that will get the actor some experience, providing the role is a character that the actor is suited to play. Often, actors will not know the roles they should be playing and there is often a divide between the characters an actor wants to play and the ones they will actually get hired to play. We will cover this as well.
After doing a handful of short films, student films, deferred payment, no payment parts, an actor should know when it is time to move on to projects that are more fulfilling, not only financially, but also artistically.
Once an actor decides they do not need to submit for anything and everything, and it’s time to be more selective about the types of characters they will play, they then need to know where they fit.
Nobody wants to be typecast or limited in terms of their choices, at the same time, it is vitally important to know where one belongs and what roles one is best suited to play. It becomes easier for actors to explore and other characters which are against type once they have established themselves.
There can be an enormous chasm between the roles the actor wants to play and the roles they could/should/might be able to play.
One of the more valuable lessons I have learned as a screenwriter is how to create interesting and believable characters, not that I always succeed, but I do have a helpful roadmap along the way. This roadmap is in the realm of the archetypes. The word archetype means “An original that has been imitated.” While no one wants to be an imitation of anyone else, there is no denying that when we enter a room, our appearance, attitude and behavior will inform others about who we are. Whether entirely accurate of not, our bodies have spoken.
Others will quickly, often unconsciously, decide if we are a charmer, librarian, boss, nurturer, seductress, bad boy, etc. It is not a judgement but it is an assessment.
It gets very confusing when an actor sees themselves one way, the world sees them another way, and the roles they long to play are completely different.
For example, if an actor is often compared to Jennifer Aniston but she thinks she is the next Meryl Streep, a little self-examination might be in order. No, it’s not impossible, but it is highly improbable that this actor will have the career of a Meryl Streep. It is so important to know the threads that you bring to the fabric being woven.
The studio conducts a workshop on occasion about the archetypes and it helps give actors a clearer idea of the roles they could play, might play and will never play. It’s based on comparisons their friends and family make as well as their classmates. It is incredibly helpful to ask those who know an actor best to share the actors or characters from film, tv and stage that they think are comparable.
While all art is subjective, there can still be an agreement of what pieces of the puzzle fit together.
As Socrates said “To Know Thyself is the Beginning of Wisdom.”