MC Depths

I'm not to sure about this MC depth thing but I think it's causing problems to a game I'm making.

If 2 movieclips, completely different from each other have been created by different movieclip actions but have been given the same depth, does this mean one will over-write the other?

Also can you assign a depth of -1 for example so appears under a movie clip with a depth of 0?

Thankyou for your reply if you do. Great help.

Only one mc / depth and Ive seen many posts here using negative values for depths.