So instead I thought I'd talk about a sister topic to Characterisation.
In general - unless you're pitting your protagonist against nature in some way - your antagonist is a character. In fact they are the second most important character in your story because without someone to get in the way there wouldn't be a story. Therefore they come with the same need for development as any important character.
When developing your antagonist you need to consider the following things:
- Motive. Why does your antagonist get in the way? No one thinks they are evil. Your antagonist needs a reason that makes some sort of sense. It can be twisted but it should be comprehensible. BTW if your antagonist's motive is immortality - think very carefully about it. Why does he do things no right minded person would do for this goal? In short why does he want to be immortal? It's not even impossible for the antagonist to be right - though this can lead to a total downer ending if handled badly (unless you want a downer ending which is perfectly valid).
- What are their good points and how can you show them off? Unremitting evil is boring - and in humans at least unrealistic. If your antagonist is Satan, Great Cthulhu or similar then okay its fair that there are't any, but if you do have a supernatural big bad consider keeping him or her in the background, and having them make their presence known through their human servitors.
- What is their inner conflict? Just like your protagonist your antagonist needs inner conflict.