Martin (1992: 131):
The last transitivity role to be considered is Circumstance [sic] of Role, which like the Attribute, simply functions to classify participants, not introduce them (Halliday 1985: 142):
[3:69] She arrived as Dracula
Actor Process Circumstance: role
 To be clear, Martin has defined 'participant' in terms of nominal groups functioning as the Agent or Medium of a clause. This definition already excludes all circumstances.
 To be clear, Rôle construes the participant Attribute or Value circumstantially. Halliday & Matthiessen (2014: 326):
This category construes the meanings ‘be’ and ‘become’ circumstantially; the Role corresponds to the Attribute or Value of an ‘intensive relational’ clause.
 In SFL theory, nominal groups inside prepositional phrases have the status of indirect participant (Halliday & Matthiessen 2014: 312). And, even in Martin's terms (ideational denotation), such circumstances can "introduce a participant" whose identity can be subsequently presumed, as shown by:
As Dracula she arrived. He had always been her favourite fictional character.
 This is very misleading, since it falsely implies that Halliday endorses Martin's misunderstandings of Attribute and Role, as well as misunderstanding of textual reference as ideational denotation ("introducing participants").