Commissioning Process.

An agent is a person who helps a writer to find job opportunities, acting on the writer's behalf to generate interest in them, receiving between 10% and 20% of the profits generated by the script in return. If works comes to someone through an agent, it's taken more seriously than just by sending a script. It also gives the writer more time to work on the things they write, whilst the agent is out trying to generate publicity and work for them. It's easy to find agents for screenplays through books, or sites like this:
http://www.jengovey.co.uk/screenwriters_friend/screenwriter_agents_uk.html

A commissioning editor at somewhere like the BBC would decide what gets made into a television show, film, or whatever it is. It's an important step in the process if say, you wanted to turn a script into an ongoing drama on the BBC. Rather than waiting for a commissioning editor to find a script and contact you, a writer could submit a script through an e-commissioning process and it enables content makers to submit their proposals for an commissioning editor to pick up the work and run with it. Lucy Richer is a commissioning editor for independent dramas in the BBC, and she is the sort of person I would have to contact if I were interested in making a drama for the BBC.
http://www.bbc.co.uk/commissioning/whos-who/tv/drama-films/drama/lucy-richer.shtml

If the company isn't interested, you could interest another company in, say, Warner Brothers, in turning it into a film instead. If something gets greenlit, it's a go production and it will happen for sure. To get this to happen, it needs to get seen by a producer or president of creative development. Paul Broucek is the president of Warner Brothers, for example, and he decides what sort of films the company will be making in the upcoming year, and would overall control the producer that considers turning the screenplay into a film.
http://www.warnerbros.com/studio/executives/divisional-executives/paul-broucek.html

If then this doesn't get interest, a smaller, independent film company can possibly take an interest and decide to make the production. However, the script writer can either be involved greatly with the production, or barely acknowledged at all. It varies depending on the contract, company, and script writer's status. Some screenwriters report that they were completely shut out from the production after their script was bought by the company, whereas some writers - such as George Lucas - was involved intensely with every step of the production.
http://johnaugust.com/2003/writerdirector-disagreements

Errors and omissions insurance (commonly shortened to E & O Insurance) is to protect the company if you have accidentally used someone else's copyright without being aware, or using a name based off someone you know, or character names that they have no rights over, or anything that could get the company into legal trouble for example, it would protect them, and cover them from getting a hefty fine. There's such things as fair use in the United States, but it varies in England, so errors and omissions insurance can be quite important to smaller, independent film companies. These can be found and purchased commonly online through sites such as these:
http://www.hiscox.com/small-business-insurance/errors-and-omissions-insurance/e-and-o-coverage/