In teaching acting, or in giving workshops, I also teach feedback, or specific criticism.
In the Intro to Theater class I teach, the students are required to attend three plays and write reviews of these plays.
My instructions are to choose an aspect of the production, such as acting, directing, scenic design or costume design, and write about how that aspect aided or did not aid in that production's realizing the vision/message of the play.
Each semester, I find ways to further refine instructions to guide students to seeing and evaluating the performer's art.
I am trying to get closer to providing a template for meaningful criticism of the production, not the play; of these particular actors' performances, not the characters.
I try to guide students to choose an aspect of the production, such as scenic design or costume design, where choice of line, color, texture, pattern are easier to distinguish. Do color and shape set a mood of light-heartedness or shadow? Does the costume tell you where the play is, in time and space? Do the setting and costumes work together to tell you if the play is striving for realism, abstraction, or fantasy?
It is easy to get drawn in by the acting, but harder to critique the actor's work. Like the director's work, the actor's work is ephemeral, hard to describe, hard to find that specific moment and describe what made it work.
For me, the best criticism gives the theater artist constructive information about what is suceeding and about what needs improvement in the creative work. While it is nice to hear "You were great!" such grand, vague comments give one no more to build on than a general negative comment.