It has to do with his fighting stance. RVD's stance will give you the chair dropkick, as well as a slingshot leg drop on the apron. I think I mentioned how to do that before, but you have to whip your opponent into the ring when you're standing right next to it, and their head will be sticking out over the side. Press O and any direction to do it. That same thing with the Undertaker 1 stance will do his apron leg drop.
The stances also affect the way you sell strikes, Brock's basically no sells everything, Jericho hops around comically, Mysterio adjusts the mask, etc.
A couple more hidden moves: If you have swanton 2 as a finisher, an up or down grapple on top of a ladder will result in you pushing your opponent off, doing a different, extended taunt, and swantoning onto them. If you have Walls of Jericho 1 for a finish, you can do that on the ladder in the same way. If you have the 619, you can do a manual 619. If they're leaning against the ropes, run at them and press X by itself to do it, it's the best way to set up the springboard leg drop.
Oh, and the zombie situp can't be done by anyone except old school UT, no matter what you do.