I recommend both pieces: "From Affection to Soul," published last year in Charles Stivale's collection Gilles Deleuze: Key Concepts; and "Fashioning a Stave, or, Singing Life," from Jennifer Daryl Slack's Animations (of Deleuze and Guattari).
Seigworth locates a concern with affect at the centre of Deleuze's work: "It is an implicit (and sometimes explicit) movement through the vicissitudes of affect that continually guides Deleuze's thought" ("From Affection to Soul" 166). He continues by usefully distinguishing between three facets of affect as Deleuze takes them from Spinoza: affectio, that is, "an affection of a body by or upon another" or "affect turned 'effect'"; affectus, "affect as a line of continuous variation in the passage of intensities or forces of existence" or "affect as 'becoming'"; and affect "as entirely active or as absolute survey," i.e. "pure immanence at its most concrete abstraction from all becomings and states of things" (166-167).
These three aspects also outline a methodology, a mode of both investigation and praxis: from a materialist conception of social interaction in terms of the power and effect of bodies in resonance or collision, to the discovery of lines of flight and processes of becoming, to the plane of immanence and "the attempt to grasp power positively not only as an effect or in its effects" (168). The development implicit here is a "series of beyondings":
an increasing expansion or widening out: from the affective capacity of bodies (corporeal or incorporeal) to interval (as place of passage between intensive states or continuous variation) and, finally, to plane of immanence: as "the absolute ground of philosophy." (168; the quoted phrase is from What is Philosophy? 41)But Seigworth's "Fashioning a Stave" complicates what would otherwise be some kind of teleology, by showing how at any point along this line affect can either be interrupted, by signification, or fold back in upon itself, to become habit.
Classical, oedipalizing, psychoanalysis presents a paradigmatic example of affect's arrest, its limitation by representation. "Here," Seigworth observes of Freud's famous account of a child's game of "fort/da," "the motility of affect, with regard to the materiality of unconscious processes, is brought to a standstill" (n.p.). Freud conjures up the signifier "fort" from the child's refrain "o-o-o-o," interested less in the resonances of vocalization than in the establishment of a symbolic order. And where Freud maintains some ambivalence about this manoeuvre, Seigworth shows how Lacan takes it a step further, banishing affect altogether as a "term one must completely expunge from our papers."
But its interruption by signification is not the only fate that can befall affect. It can also become the basis of a habituation. This is the reverse of the development described above: rather than taking us from affectio to affectus, here "affect (affectus) turns to affections (affectio) while affections, as the anticipation of interbody foldings, produce habits." Seigworth emphasizes the way in which habit feeds back into becoming, the way in which "habits come to serve as the ground, the scrap of familiar territory, that then provides a motor [. . .] for all future becomings." But one might equally point out that habits can stand in for becoming, can forestall becoming as surely as does representation (but now immanently, without ever rising to the level of signification or ideology).
This is the point at which I turn to Bourdieu, for his understanding of habitus as a mechanism that ensures social reproduction at a level well beneath the ideological. Which is not say, as I've mentioned before, that one couldn't or shouldn't imagine other habits, habits befitting new forms of liberating, multitudinous, subjectivity.
On the other hand, as I've just tried to point out in a rather different context, there's no necessary value to be attached to affective intensity.
Habituation and affectation: these are the conjoined processes that posthegemony theory should study, not to valorize one over the other, but to chart how they combine to produce varieties of subjectification, sometimes constricting, sometimes empowering.
[Update: I have fixed the typo that I introduced to one of Greg's article titles; thanks to Greg for pointing out the error, perhaps symptomatic on my part. I had put "affectation" for "affection." Maybe more on this anon, but despite the poor connotations that the word carries with it, I like the idea of "affectation" to indicate (in parallel with "habituation") the process by which affects are produced, or by which other processes become "affected," reveal their resonance or grounding in affect.]