Audiences judge an actor's performance by several criteria, all of which are dependent on theatrical and other social conventions.  Today, in most kinds of performances, we say that an actor whose performance we enjoyed was "believable."  This means that his or her actions, movements, and voice were appropriate to the character, plot, and performance style of the play.  When we consider virtuosity in acting, we tend to think of actors' abilities to transform themselves, so that they appear substantially different from role to role. In some performance styles, virtuosity depends on other aspects of an actor's performance; for example, a musical comedy actress such as Carol Channing does not transform herself significantly from role to role, but she delivers songs and dances that are tremendously entertaining.

The actor's secondary goals are also important.  First, an actor's performance should draw audience members into the theatrical illusion being presented on stage.  The energy projected by the actor begins the cycle of feedback described in the last section:  the live audience responds to the actor's performance and the actor in turn adjusts his or her performance in response to the audience.  Second, since acting is usually a group art form, the actor must work within stylistic constraints established by the playwright and director, and the actor must work effectively with the other actors in the cast.  We often refer to "chemistry" between actors or a "strong ensemble" when many actors work together effectively.

Tools
An actor's tools are unusual among artists.  Whereas a painter's tools are his  watercolors, brushes, and paper, and a pianist's tool is her instrument, an actor has no tools but him or herself.  An actor uses voice, body, and mind/spirit to create a character.  Like the pianist or other performing artists, the actor is usually an interpretive artist, meaning that the actor is part of an interpretation of a primary artist's work -- in this case the primary artist is the playwright.  Because an actor is usually interpreting a role written by someone else, the play text is also a tool for character creation.

The actor usually begins with an analysis of the play:  he searches for all clues about the character's past life, present actions, relationships with other characters, and function within the plot.  From there he can begin to create the character.  Anything not supplied by the playwright directly is up to the actor's imagination.

The actor then uses her own body, voice, and mind, adapting aspects of herself to become the character.  The actor may alter her voice by varying her pitch range, dynamic range, timbre, and tempo or by speaking with a dialect.

The actor will also use his body in ways specifically chosen to express his character.  He will select gestures, postures, and movement patterns that may be different from his own.  Some of these physical aspects of the character may be determined by the customs of another historical time;  for example an upper class, male character from the English Restoration may gesture with a handkerchief or fan for effect, carry a snuff box, and stand with his legs turned out at a ninety degree angle and the weight resting slightly into one hip. A woman, like this character played by Ann Bracegirdle, might carry a handkerchief of parasol, but her posture would be similar to a man's.

The character's mental processes are also important;  these include thoughts, emotions, and psychological traits.  Analysis of what a character says and does in the script of the play will be an important beginning for an actor in finding her character's "inner life".