That first sentence (or two) is critical. They must make the reader keep reading because they HAVE to know what’s going on. That means starting “in media res,” in the middle of things. Like these:
“The man with ten minutes to live was laughing.”
The Fist of God by Frederick Forsyth
“The small boys came early to the hanging.”
Pillars Of The Earth by Ken Follett
“For a week, the feeling had been with him, and all week long young Paul LeBaeau had been afraid.”
Iron Lake by William Kent Krueger
Bear in mind, you don’t have to sacrifice humor.
“Ricki Feldman is best admired from a distance – if you get too close, you might find some of your body parts missing.”
An Image of Death by Libby Fischer Hellmann
“My bodyguard was mowing the yard wearing her pink bikini when the man fell from the sky.”
Dead Over Heels by Charlaine Harris
And my all time favorite:
“I turned the Chrysler onto the Florida Turnpike with Rollo Kramer’s headless body in the trunk, and all the time I’m thinking I should have put some plastic down.”
Gun Monkeys by Victor Gischler