It is important to separate Cast from the physical email you drop in.
As Val points out, a Cast is a time/date that the system will send the next email - for THAT lead.
So if the next Cast is Dec 10 at 815am, Marketo will send all of the emails it has out to those who are NOT Exhausted or Paused and in that Stream. All this means is each lead has a schedule:
Lead 1 just joined and his next email is Email 01 on Dec 10
Lead 2 has already received Emails 1,2,3 and is due for Email 04 on Dec 10
Lead 3 is Exhausted, received all the preivous emails and will NOT receive anything at the next Cast. This can be changed if you add a new Email to the Stream.