japanese question(s)

when does someone use

watakushi instead of watashi

and when do ppl write watashi in hiragana instead of the kanji symbol

and when do people say san in katakana instead of hiragana...

cos ive seen these questions on youtube trailer of p3p... and is it just me, or did they make mistakes?

im expecting Shinobi_Neo to answer me hopefully, cos i saw on one of her blogs she knows some jap