Let’s say I am a budding sitcom writer and I get hired onto a TV show that has been running for 5 seasons. Unless I have sat down and watched every episode there are aspects of the story or characters I may not know about or fully comprehend.
Before I begin writing do I have to watch every episode or does someone bring me up to speed on the story so that I can just start writing? Does someone then take what I write and meld it into the story so it appears seamless? And what does writing a sitcom script on spec mean?
Basically, you write a sit-com script of an existing show (that’s not going to hired you on staff, in all probability) to get an agent interested in representing you. (This is itself an essential part of the process, getting an agent.) If successful, you may get a job at that point as a “writer’s assistant” on a different sit-com, and eventually you may get offered the chance to submit a spec script on a specific theme (or sub-theme, meaning you’ll get asked to write a scene or two). If that’s successful, you may get promoted to “writer” and be asked to submit further scripts. There’s almost no chance that, without a good Hollywood agent, you can write an entire script for an existing show that will get read, much less produced.
Some shows, dramas, usually not sitcoms, take manuscripts from unassociated writers. Sometimes a good story will be reworked to fit a show. As pseudotriton ruber ruber points out, without an agent, or a personal relationship with a producer, you have no chance of have a script picked up. If you go to LA, you will find that everyone has a script. No producers will bother to read any of them without a personal recommendation.
The showrunner is the person responsible for wrangling associated writers and what they produce into a coherent show narrative and maintain continuity.
I recommend reading Ken Levine’s blog; he was a writer for MASH, Cheers, and Frasier amongst others. He writes a lot about what it’s like to work on sitcoms.