Hello.
(1) Has anyone tell me about the process/ schedule of a sitcom
from the script to the screen?
I'd like to know when a TV station decides to broadcast or film a pilot.
(2)Is a pilot filmed by the different actors/actresses to the real show?
Thank you 
The real second question is "Do the actors from a pilot end up in the series?", to which the answer is: not necessarily.
Many years ago, pilots tended to be a test of whether the public liked a show. As such they were often near-indecipherable from a series proper, featuring all the same cast and characters. See Are You Being Served? as a good example of this. A few decades ago, there were even regular, hugely popular seasons of pilots. Some of Britain's most popular shows - including AYBS? - came from these. See Comedy Playhouse.
However, now pilots tend to be more to test the whole concept of a show, its format and dynamic. It's relatively rare that they're even broadcast. Some will have the same cast, but others mightn't even have the same characters. See Coming Of Age for this.
The key point to remember is that a pilot script will be written with, if not individual actors, then certainly particular on-screen qualities in mind. And, as a pilot's purpose is to convince those in power to commission a series, the writer, producer and director are going to get as good an actor as they can for each role - not just "anyone'll do". Therefore it's natural in many cases that the talent would be invited to participate in a commissioned series. Whether they agree or not is another matter.
It's difficult to add much to what Aaron said.
The only thing I would also say is that pilot episodes can often be made by independent companies and may not be associated with any specific TV station. Some of these can be made for very little money and the production standards will often not be close to those of broadcast TV shows.
Dumb question.
Why are they called 'pilots' ?
Quote: lofthouse @ July 22 2011, 6:10 PM BSTDumb question.
Why are they called 'pilots' ?
I think it derives from nautical usuage. A pilot was distinct from the everyday captain in being the person who steers the ship in difficult situations, like getting in or out of a harbour. A pilot programme is also a guide.
Not funny, but you learn something.
Quote: lofthouse @ July 22 2011, 6:10 PM BSTDumb question.
Why are they called 'pilots' ?
Because if they don't go down well the suits wash their hands of them.
Thank you, Aaron & Nick!
I understand the situation in the sitcom has changed.