In screenwriting, which I also do, a common mistake of beginners is to begin the story too soon. You've got about 5 minutes to win the audience over, and if the script dawdles with background material you've lost them. Of course, a script like that would never get past the studio reader, who has even less patience than the ticket-buying public. If a screenplay doesn't show promise by the bottom of page one, it's tossed. There are exceptions, but they don't apply to unknown writers.
Another rule of good screenwriting is to have plenty of white space in the script. Before a script is even read it's flipped through to see if there's plenty of air. Too many words and the reader won't even bother with page one.
Now, combine the white space rule with the requirement of hooking the reader by the end of the first page, and you get an idea of how careful a script writer must be with his or her words and scene selections.
Novels can contravene both rules -- they neither have to begin with a punch nor follow the path of tight writing. Yet my personal preference is to get the reader on my side as soon as I can. Here are two novels and their openings that won me over immediately:
1. The Andromeda Strain, Michael Crichton, 1969, fourth paragraph:
"There is something sad, foolish, and human in the image of Shawn leaning against a boulder, propping his arms on it, and holding the binoculars to his eyes. Though cumbersome, the binoculars would at least feel comfortable and familiar in his hands. It would be one of the last familiar sensations before his death."
2. The Chamber, John Grisham, 1994, opening sentence:
"The decision to bomb the office of the radical Jew lawyer was reached with relative ease."
Notice, too, that Grisham opens with a sentence in the passive voice and does so quite effectively.
Of course, one could argue that neither book is an example of great literature, that many engaging and deeply moving stories avoid having to hit readers over the head to get their attention.
Still, I liked these openings and the stories that followed.