Run-through rehearsals are the most interesting phase of an opera production for many pianists. After weeks of working on the individual scenes, the opera is finally played as a whole. This allows the pianist to better understand the musical context of the opera and to better coordinate the individual scenes with each other.
I am convinced that a good performance in an opera requires not only the mechanical singing of phrases, but also a precise understanding of the musical whole. This is especially true for operas that consist of several, independent numbers. Unfortunately, many singers are not sufficiently interested in the music of their colleagues and focus only on their own role.
The vanity of the stage situation often leads to egocentrism and star cult. This is a general problem in the world of opera, which has been evident in previous centuries.
The current change in the opera business is leading to the disappearance of permanent opera ensembles. Instead, agencies form groups of singers who work together on a project-by-project basis.
The project planning includes a joint rehearsal after the search and selection of the participants. The order of the projects is initially based on the circumstances. As soon as a majority decides on a piece or as soon as enough participants are available for a particular piece, the work can begin.
A common deadline promotes group dynamics and reinforces the motivation of the members.
Not all operas are suitable for this collaborative work. It is important to avoid too great differences between large and small parts. Therefore, it is tried to upgrade and enlarge smaller roles in order to avoid hierarchies.
Traditionally, small roles also have a pedagogical function in theater work. Those who want to build their self-confidence on stage and do not dare to take on larger tasks, are satisfied with short appearances. (I would avoid extreme cases like "La cena e pronta" in Traviata...)