Snape was the most complex of all the characters, and also the bravest, most loyal and the strongest.

One of the changes they made for film, which entirely undermind his role in the story, was Harry breaking into Snapes mind to witness his memories, rather than sneaking a furtive peek in the bowl, as he in fact did.

Snape's key role as secret agent was entirely predicated on his exceptional skill at defending his secrets from Volly (V being heralded as the most powerful legilimens in existence, if memory serves) of course!

It also pissed me off that the film team chose to edit out Harry's entirely human furtive spying episode, in favour of making him even more heroic and brilliant at Snape's expense.