Focusing on specifics:

* Particularize

* Specify

* Detail

* Individualize

* Differentiate

* Distinguish

* Categorize (in the sense of creating very specific categories)

* Itemize

Focusing on individual cases:

* Personalize

* Individualize

* Customize

* Tailor

Focusing on complexity:

* Complicate

* Nuance

* Qualify

Other antonyms:

* Specialize

* Refine

* Refute (in the sense of disproving a generalization)

The best antonym will depend on the specific context in which you are using the word "generalize."