I loved the idea of snakes providing more companionship and acceptance to Snape than people in his life ever did - so they had to have distinct personalities, as any large human family would.
It was a key point in the story that there's a hell of a lot more to the character of Snape than a sexy voice: I didn't see that he would be any less fascinating a man without it. He's resourceful enough not to let such a thing defeat him.
At eleven, seventeen, or twenty-seven Harry will always be an adventurous, curious kid at heart. That's why the idea of him exploring the wider world was so interesting to me.