Promoting one participant, demoting another
From the point of view of the textual organization of what the speaker wants to say, it follows that any of three possibilities may condition the choice between active and passive:
1 An element which is not Agent is desired as Theme/Subject/Topic.
2 The Agent is New information, so will be placed last.
3 The Agent is not New and is silenced. Some other element is New and is placed last.
It is not simply a change of position that is involved in the re-structuring of the passive clause. It is also a question of topic promotion and demotion. In the active clause, the Agent–Subject has the discourse role of Topic. That is, it is the most important participant of the discourse at the point when the clause is produced. In the passive clause, the Agent ceases to be Subject/ Topic. Another participant (usually the Affected) takes on the roles of both Subject and Topic. The Recipient can also become Subject in a passive clause, as in The boy was given a mountain bike for his birthday.
The demotion of one participant and the promotion of another are two sides of the same coin. If we demote the Agent (or Experiencer, or Sayer), then a different participant (Affected, Recipient) is automatically promoted to Subject. It is clear, therefore, that, first, the passive is not a type of fronting or thematization; second, it does not produce a marked Theme, but a different unmarked Theme; and third, the type of Theme involved is a participant Theme, which we call Topic. Circumstantial Themes and textual Themes are optional additions to the core clause and play no part in restructuring the clause as passive.
We now turn to the discourse motivations that involve the choice of passive. Basically, these are: to cut out unnecessary Given information; to manoeuvre important information into end position; to establish smooth connections between clauses, making for good information flow. These motivations work together in connected discourse. Choices of passive against active are not open, but are conditioned in each individual case by the immediate contextual environment.
