Jackal surprise
Expanded August 5th 2007
Try this on Heroic or lower. After leaving the final Marines behind and riding up in a lift, there's a Jackal waiting at the top. Try to kill him quick without his colleagues getting alerted. A few melee attacks are probably your best bet, though gunfire doesn't always cause an alert. You'll get a checkpoint once he's down, and that'll be handy to revert to for replaying the fun coming up. On Legendary there are two Jackals you'd have to kill stealthily. That would make things harder to set up, which is why I recommend Heroic or lower.
On exiting the tunnel you emerge into a small sunken room with some supplies lying around. Two Jackals guard the short ramp that goes up into the next room. They're normally facing away from you, but if they're partly turned your way (maybe half alerted), wait until they turn back around. You can now have some mischief with them.
Right beside you
Creep up and stand next to one of them. When you're finally noticed, which can sometimes take amusingly long, you should see a great animation of surprise as the spotter almost jumps out of his skin, having realized there's a ruddy great cyborg present. Good comedy. Normally it's the far Jackal that sees you first, and his reaction catches the attention of the second, the one you're next to. While the first guy squawks and cowers defensively behind his shield, the second looks at his colleague quizzically as if to say "Well what the heck are you doing?". He doesn't realize there's a cyborg breathing down his neck. But eventually he cottons on - possibly after getting hit by fire that's meant for you - and it's another fine moment of comedy. After that, do what you like; I'm sure you can think of something!
Top marks to Bungie for making this sort of enjoyable wickedness possible; more games ought to include reactions of enemy surprise for us to enjoy. I mean, when you were a kid you got a kick out of making someone jump, right? Well, you probably did if you were a boy at least. Halo makes that same mischievous kick available, not just with Jackals of course, but also with Grunts and Elites. They all have amusing reactions of surprise to take pleasure in. Not that you have to have been an evil little boy to enjoy this game, but it probably helps!
Grenade ahead
Another way of seeing some good reactions is to lob a plasma grenade onto the ramp. Usually both Jackals will do a delayed reaction of surprise, recoiling and bringing up the shield while emitting an involuntary squawk. Hee hee. And if you placed the grenade low enough, they'll be blasted shortly afterwards. If it was further away they'll remain alert, but methinks they're looking the wrong way! They'll be looking up the ramp, not realizing what's behind them. Enjoy waking them up.
Talking of which, you can tag a snoozing Grunt at the top of the ramp instead of lobbing a grenade onto the ramp. Poor grunty. That's some wake-up call eh?
A little gift
When you get into a fight with the Jackals, it doesn't take much to get one running off up the ramp whimpering, with shield overhead. Tag him with a plasma grenade or two; a little gift for him to spread around among his friends. There's a pause and then the glorious sounds of carnage. Excellent.