A New Historicist looks at how the individual writer writes but also how the writer was influenced by his/her society and historical context; Point A: including influences the author would not be aware of. Simultaneously, a New Historicist must consider his/her own possible historical biases and must consider the historical and cultural differences between, say, a 19th century Shakespearean critic from Jordan and a 20th century Shakespearean critic from Mexico.
Althusser is a New Historical-Marxist. For him, identity is acquired through economic-social relations but also through ideology. Althusser stated that we are "always already" subjects; subjected to ideology. Ideologies, ideas and how to act (identity) are all material; they are rituals; subjects comply with the production of their own identities. Hence, he talks about ISA's and RSA's (Ideological and Repressive State Apparatuses), which like Foucault's panopticon, reinforce these identities. Since subjects are 'always already' subject to being watched, they police themselves; they are complicit in producing their own subjugated behavior.
The Marxist tradition looked at how the individual would recognize his/her individual subjection to larger social forces and this awareness of subjection would be emancipatory. But Althusser spins this: subjects are individuals oppressed by social forces; but, subjects are individuals produced by ideology and the relations of social forces and production. Their awareness of subjection is not emancipatory; it is recognition and reproduction.
So, just as social forces "hail" or produce subjects though economic determination and social oppression,texts and novels produce subjects as well. So, readers not only identify, relate or recognize themselves in characters and literature, but are produced by them.
This is all part of New Historicism; that history (social forces) and literature (all forms of writing) are interdependent. They write and inform each other; and write and inform the subject (individual). And they do so by basically saying, 'yes, this is us. These are our roles and identities. It may suck, but we're all in this together. We are each individuals but subjected to social forces and there is comfort in commiseration." The reader is not just empathizing; the reader is being addressed "hey, you feel this way too." The reader's identity is not emancipated as in "oh, we are oppressed in that way;" rather the reader's identity is being produced! "hey, you feel this way." Reader, "Oh, yeah, I do."
And - Like Foucault's panopticon; people police themselves because they are always already subject to being watched; always already subject to ideology. So, bringing this back to Point A, Althusser notes how the subject, like the author, may also be unaware of how he/she is producing his/her own identity via the interdependent relations of writing, history and society.
I guess his biggest contribution in this context was to note the prevalence of ideology in art and literature as well; and to recognize that writing may be ideological production disguised as liberating text/criticism.