"Every effective opening needs to do three things. The chief of these is to get the story going and show what kind of story it's going to be. The second is to introduce and characterize the protagonist. The third is to engage the reader's interest in reading on."
"Get your action rolling first, then back off to show the normal state of things."
Lawrence Block in his Telling Lies for Fun & Profit says:
" . . . most effective openings do several things at once. They get the action going, set the tone, and establish the problem--and while they're at it they may sketch a character or two, convey some important information, take out the garbage and sew a button on your cuff."
Nancy Kress in her book Beginnings, Middles, & Endings says:
"In your first scene, your main goal is to keep your reader interested. You do that through focusing not on overall meaning but on the four elements that make a first scene compelling: character, conflict, specificity, and credibility."
"Your opening should give the reader a person to focus on."... "Begin with an indication--subtle or overt--that something is not going as expected, or someone is experiencing disturbing emotions, or something is about to change."
Sol Stein in his Stein On Writing says:
"The ideal goals for the opening paragraph are to:
Excite the reader's curiosity, preferably about a character or a relationship.
Introduce a setting.
Lend resonance to the story."
Summarizing what Crawford Killian says in his Writing Science Fiction and Fantasy about the functions of an effective opening: The opening . . .