Hi Paul - thanks for writing in!
This is a great question, and one that I think can't be talked about enough.
Your goal with every single shot on your demo reel should be to show as wide a variety of skills and expertise as possible.
Obviously, that isn't possible with every shot. A fight sequence, for example, may not have an opportunity for a lot of deep character introspection and subtle emotion - it might be more about dynamic camera angles, powerful poses, and exciting body mechanics.
However, the goal SHOULD be to show off as much as you can. If it's a fight scene, then you should be showing off all of the above (camera angles/moves, dynamic body mechanics, etc) as well as showing your deep knowledge of all of the basic animation principles as applied to an action shot. We'll want to see anticipation, squash/stretch (maybe even just subtle squashes in the spine or subtle stretches on the punching arm), nice arcs, great weight & balance, etc.
If you're going to do a dialogue shot, please don't fall into the trap that so many students fall into, which is to animate the classic "guy sitting behind the desk or at a table" shot. The acting may be great, but if you do a shot like that, you better have some other terrific body mechanics shots on your demo reel, or you're missing an important opportunity to show off your skills.
Sometimes I'll see a series of really great acting performances on a demo reel, but the characters are always sitting or standing in place. Sadly, the best acting in the world is not going to catch our attention all on its own. Most studios (probably ALL studios) also need to know without any doubt that you also know how to make that character stand up and walk around!
My advice in these situations is to design your acting shots to show off both acting AND body mechanics. If you want to start an acting scene with seated characters, that's fine, but the shot will be far more impressive if you have that character stand up and DO something as part of their performance.
Any time the feet seem nailed to the floor or their butt is glued to the seat, many recruiters just tune out completely because it simply doesn't show whether or not you truly know how to animate.
And I've said it before, but it bears repeating - for the same reasons listed above, please no more 'floating talking heads disconnected from the body!' Again, the nicest lip-sync ever done is not going to impress us if we can't even see the rest of the body.