As a parent of three children, I have learned that it is in the ability to make the hardest decisions - and the ones that often have proven to be the most painful for me - that have helped my children the most, but this knowledge and understanding only comes with experience, objectivity, and if you're lucky, a great teammate (Thank God I have a wife!).
Parents should never be best friends with their kids – that's a recipe for disaster. The same goes for your screenplay; it is NOT your friend. And just as the parent-child relationship is not equal, nor is the writer-story relationship on neutral ground. Simply put, the parent is in charge. So is the writer.
When it comes to parenting or writing, you're the dictator, albeit benign. The good parent-dictator makes decisions that are for the best welfare of his or her children – never harmful, never malignant – and if that means putting that child in "lock-down", taking away TV privileges, or confiscating her computer (no more FarmVille), so be it.
Likewise, the good writer-dictator makes decisions that benefit the script. And often, cutting something out – an idea, a character, a line of dialogue, or even a great scene is necessary. But how do you decide what decisions to make, what elements to cut? The beginning writer will make mistakes – lots of them – just as the new parent does. However, experience is your greatest asset. The more you write, the better your decision-making becomes.
But when deciding whether or not to cut an element from your script, it never hurts to follow two simple but key rules: (1) if it does not reveal essential elements of character or (2) if it does not move the story forward, then you have no choice but to cut it, or as I like to say: "KILL YOUR BABIES!"
Of course, I'm not advocating the genocide of your screenplay population - you can always use that baby in another story - but you must be able to see the forest for the trees. Screenwriting is all about the Three C's: telling a CREATIVE story while being incredibly CLEAR and CONCISE. Simple is good. And often, the best stories are the simplest seeming ones.