Although at the start of Seminar X he says little about these two concepts, Lacan situates acting out, as well as the related concept of passage to the act, on the three-by-three matrix that he opens this Seminar with (19.12.62.):"What Does Lacan Say About Acting Out?"
It is only later in Seminar X that Lacan returns to the above matrix when discussing what he describes as the passage a l’acte in the case of Freud’s patient known by the unfortunate moniker of ‘the female homosexual’. She takes a walk with the woman she loves in the neighbourhood where her father works; he sees them and shoots her a menacing glare. It is at this point that she experiences what Lacan describes as a “moment of greatest embarrassment… with the behavioural addition of emotion as disorder of movement”, as per the matrix (Seminar X, 23.01.63.) This emotion, coupled with a disorder of movement, realises what Lacan calls the two conditions for the passage a l’acte which takes the form of the jump onto the railway line. For Lacan, this passage a l’acte reduces the patient to the status of object a:
“What comes at this moment to the subject, is her absolute identification to this a [object a], to which she is reduced. Confrontation with this desire of the father upon which all her behaviour is constructed, with this law which is presentified in the look of the father, it is through this that she feels herself identified and at the same moment, rejected, ejected off the stage” (Seminar X, 16.01.63.).
The desire of the father, in other words, is condensed into the gaze, his scornful glare which precipitates her suicide attempt. Unlike acting out, the passage a l’acte of which this scene is an example is not simply demonstrative. It does not make an appeal to the Other like acting out does.