You get HLMV (www.google.com) and open the models from the pak0.pak file in your Sierra/halflife/valve or Sierra/halflife/valve, I can't remember which, in HLMV.
Then you search through their animations until you find ones that have that animation.
A model is just that, a model of something, made up in much the same way that Hammer works, but far more complex and on many more angles.
To make a model do anything you have to animate it. Not all teh models have all the animations, that would have been time consuming to do, so models such as barney have a larger number of animations than an alien slave for instance, as you'll see him do more things in the game.
HLMV is a Half Life Model Viewer, and amongst the Tabs at the bottom there is an Animation tab, use this to select the different animations a particular model has.
Then you use airscripted_sequences or something equally ghey.