Well...Few things to put in perspective.
"Demon" is not only just a grossly over-used word, but it is also relative to the situation. It depends on who you're asking and what a "demon" is to someone. Some people and cultures have a far more forgiving scale to judge by than others, other cultures loosely label anything that is of negative influence a Demon or Demonic in Nature.
So... Many beings are unfairly labeled as demonic. 90% of what people encounter in any real not-so-pleasant metaphysical experience is not an actual "Demon" in the truest purest sense of the word.
Demons are NOT "evil".
Real Demons are largely apathetic towards humans. Real Demons don't seek you out, you seek them out. Not just metaphorically but literally as well. The "Lesser" kind usually take interest out of habit to come to you lol.
Demons deal with darker aspects of the human psyche. You banter with them, they will force you to live up to it. They are not your "buddies", they are more like an employee you have need of when the time comes. Demons are completely "Fluff free zone".
So To answer your question on how they may appear, it depends on the background. They are quite grotesque, raw, and nasty. Consistently Ravenous for chaos. These are the type of "real" demons that have no real thought process of their own. They have no emotional range at all; the very likes we cant comprehend at the very least, so they are somewhat zombie-like in that aspect.
Once you get past that point, the other types that may be loosely labeled as "demonic" are those of a better nature, and they carry infernal qualities that may be "perceived" as demonic. Just because it looks "scary" and "evil" doesn't mean it is...
But remember they have free-will also, so it's completely up to them how they want to approach you. Depending on there nature, thought process, instinct, personality, they should be judged individually, and as a race on there own collective merit.
That's basically it, take it for what it is.