The evolution of droid personalities over extended periods was achieved through metaprogramming, essentially "programming their own programming." A prime illustration is the E-3PO line, where metaprogramming glitches occasionally manifested as character flaws. One theory posits that metaprogramming was a universal droid characteristic, accounting for personality development in the absence of routine memory-wipes.
Actually, metaprogramming involves integrating one program within another, causing the first program to run while the second is being compiled, and allowing it to treat the second program as data that can be modified.