Stuntman: Ignition

While you can get 1,000,000 points on nearly any medium-to-long scene if you do it right, I scored between two and three million points on each of these scenes:

Aftershock: Scene 6
Whoopin' and Hollerin' II: Scene 5
Strike Force Omega: Scene 6
Overdrive: Scene 4
Never Kill Me Again: Scene 2
Night Avenger: Scene 5

The key to getting a million - or more - points is keep your score multiplier high and your stunt string unbroken: the more stunts you do one after the other the higher your score multiplier goes; break the string and the multiplier resets. You can't go any longer than two seconds in between stunts to keep the string alive.

To keep your string going, keep doing the director stunts and drive as close as you can to objects on the courses without having to go too far out of your way: boxes, other cars, trees, telephone poles, benches, etc. Not only do these keep the stunt string going, but your multiplier increases too. So keep doing it.

Another way to keep the string going is to do drifts, and wheelies if you're on a motorcycle. It doesn't matter how brief they are; just keep that string alive.

This achievement takes practice and a bit of luck, but the above-mentioned scenes are probably your best chances at it.