Control default display settings for custom CCK field

I'm developing a custom module that provides a custom CCK field. In my module, I would like to define the default settings for the display of the field in teaser and full node and also of its label. I supposed that it's possible in theme_<module>_formatter_<formatter_name>, but I can't figure out how.

What I need is that by default the label is hidden and that the field only appears in full node, but not in the teaser.