The software you choose to write your scripts comes down to a matter of personal taste. If, indeed, you choose to use software at all. Some writers still prefer the good old fashioned pen-and paper approach. Stephen King often writes his novels this way. Hard to argue with that one.
Broadly, your choice falls into two categories: purpose-written scriptwriting software, or general purpose Word Processing software.
F ar and away the most popular choice. Almost all screenplays are written with Final Draft, or imported into Final Draft at some point in the production process.
On the plus side, it offers many templates, taken from existing shows, movies and plays, including the 'house style' of all the major studios and networks. It has nifty auto-complete options, to save your poor fingers from RSI, filling in character names, scene locations and even goes as far as guessing who's going to speak next when you're writing dialogue. On top of which, it offers many benefits for the production process – scene breakdowns and the like. It has an index card system for planning out your masterpiece before you start writing. There is a feature in the latest versions called 'Collaborwriter' which theoretically enables you and a writing partner to work on a script simultaneously, though hardly anyone can get it to work (try Googling it).
On the negative side: Final Draft is a bit of a fascist. If you want to do something out of the ordinary with your scripts, it simply won't let you. It doesn't allow images of any kind, because including pictures in your script is frowned on in Hollywood, so you can't add a logo, a descriptive diagram or even a splash of colour if you feel the need. It's also very sassy about 'activation'. You're allowed to have a working copy on two machines, and that's it. If you have any kind of computer disaster, or buy a new one, be prepared for a long wait or a long transatlantic phone call to get Final Draft working. If you upgrade, your previous version, which you paid good money for, is automatically de-activated. Lovely.
This is a FREE Final Draft-like program. It offers most of the features that FD does – the plus version, at $9.99 even more so. You can also purchase a mobile version for the iPhone for £2.99, though writing a screenplay on an iPhone is an activity that should be reserved for one of the less glamorous circles of hell. Theoretically, it can save in a Final Draft-like format, and even open Final Draft files, though I've occasionally had problems with both of these activities. Still, if money's tight, this is definitely the way to go.
Movie Magic Screenwriter is cheaper than Final Draft, but with all the fascism intact, and then some. You have to have a copy of the original CD permanently in your computer's drive if you want to run a secondary copy on your laptop, for instance. It has special features for budgeting. Like we care.
There are other attempts at scriptwriting software out there, but these are the big players. If anyone out there has a favourite, by all means let me know, and I'll give it a whirl.
I've used this for most of my writing career. Version 5, way back in the eighties, was perfect for scriptwriting. Microsoft spend the next 20 years steadily making it worse. The latest incarnation, however, (Office 2010 for PC and Office 2011 for Mac) is very good.
It is Microsoft, however, and it has some drawbacks. Its spellchecker won't let you use rude words, amusingly – though that's true of pretty much every spellchecker on the market, as if every user is an eight-year-old child. Annoying if you're trying to do grown up writing. Word's autotype features are limited – still worse than good old version 5 – and you'll probably want to splash out on a supplementary program, such as TextExpander or TypeIt4Me (both Mac only) to save yourself some finger-bashing. Breevy is a Windows equivalent that imports TextExpander snippets from a Mac, or you can plump for the free ActiveWords.
On the up side, most people can read Word files, though if you're submitting a script, I recommend you send it as a PDF file. Everyone can read PDF files, and they maintain layout integrity across systems, which means that page 9 on your copy is still page 9 on the transmitted copy. Best of all, it's hard for other people to fiddle with your script when it's a PDF. And Word saves out reasonably good PDF versions of files.
If you're a student, or in education, or if someone in your household qualifies, you can get a much, much cheaper version of the Office suite here, legitimately.
We all have our favourite word processor, and there's not a great deal to choose between them. Apple's offering is Pages, and it's fine. If you're cash-strapped, you could try Open Office. It's a decent mimic of the Microsoft suite, there are versions for all platforms and it's utterly free. Equally free is Google Docs, a cloud-based service, where the software is hosted online. The word processing functions are rather limited, and you might be nervous about working in such an ephemeral way, but it's undeniably free.
If you want your script to look beautiful, or you have requirements that fall outside the capabilities of scripting software, InDesign is hard to beat. Be aware, it's got a fairly steep learning curve: most professional publications, both in print and electronic, are fashioned in InDesign. It's also pretty damned expensive, though if you or a family member are in education, you can get a much more affordable version here.
It's not great for actually writing the script – that's not what it's designed to do – but it will help your script stand out from the crowd. When I'm submitting a script for consideration, I usually write in Word or Final Draft, and then I'll often import the finished script or manuscript into InDesign for prettifying. No time for those niceties, though, when you're in production.