Theater stages Why does the professor say this:

Throughout the long history of theater, three basic types of stages have prevailed. Each of these styles has advantages and disadvantages, and each is good for certain types of plays. Let’s take a look at the three styles and see why each has been so successful. The three designs are (1) the proscenium or picture-frame stage, (2) the arena or circle stage, and (3) the thrust stage with three-quarter seating.
The first design, the proscenium stage, gets its name from the proscenium arch, the frame that sort of outlines the stage and separates it from the audience. The proscenium stage is also called the picture-frame stage because it looks like a large picture frame that the audience looks through to see the action of the play. The seats for the audience all face in the same direction, toward the stage at the front end of the theater.
The stage area of proscenium theaters is usually deep, and this allows for elaborate scenery and scene changes. Scenery pieces—called backdrops—are hung on ropes or cables at the rear, in layers, one behind the other. They’re called backdrops because they can be dropped into position and then raised again as necessary. The number of different backdrops possible allows the scene designer to create an extremely realistic set.
However, the proscenium arch also creates a distancing effect—which is an advantage for certain types of drama, but a disadvantage for others. The proscenium works really well for large-scale productions with lots of characters and action, but for some plays—for, example, close-up views of real life— the proscenium is sort of a barrier between the actors and the audience.
Next, the arena stage. In the past half-century, there’s been a movement to bring theater closer to real life ... a movement toward more informal, more intimate theater. Thus, we have the arena stage—also called the circle theater or theater-in-the- round. In the arena theater, the stage is in the center of a square or circle, with seats for the audience around it on all sides. The actors are in the center, so everyone in the audience can be close to them. The effect is a sense of intimacy, a kind of closeness that comes whenever people form a circle.
The arena is the oldest style of theater. It’s interesting to note that almost all early theatrical performances were "in the round," as ceremonies, rituals, and dances in all parts of the world took place in a circle.
One disadvantage of the arena theater is that, while its design allows for intimacy, it also makes it impossible to have elaborate scenery or scene changes. This lack of flexibility is one reason why the third style of theater has become very popular again.
The third design—the thrust stage—falls somewhere between the proscenium and the arena styles. In the thrust stage theater, the audience sits on three sides or in a half-circle, around a stage that sticks out into the center. Behind the stage is a stage house from which the actors enter and exit, and scenery is changed. The thrust stage is kind of a compromise between the proscenium and the arena styles. It combines the scenic features of the proscenium theater with the intimacy of the arena stage.
The formal English theaters of Shakespeare’s day were thrust stage theaters, and today it’s the most widely used of all the designs. Its advantages are obvious. It has the intimacy of three—quarter seating around the actors, and the stage house at the rear makes scene changes possible. Also, the fact that so many great dramatic works were written for it gives the thrust stage a prominent position among the other major forms.