With a normal airline, you have pilots sitting on "reserve" at bases, who can be called in at any time to fill in any gaps that may occur. They are being paid but are not flying, it's quite a good gig if you can get on the reserve list.

I don't know how this is handled at Southwest, who does not fly hub-and-spoke and thus doesn't have a bunch of pilots sitting reserve around a base at, say, Atlanta.