Halliday & Matthiessen (1999: 99):
Relating to the textual metafunction: participants and circumstances can both readily serve as Theme (though there potentials differ); processes only rarely, other than in imperative clauses. Participants and (more restrictedly) circumstances can serve as referables identified by referring expressions, but processes cannot — as with WH- interrogation, they have to be construed as a Range together with the pro-verb do as Process: do it/that.