I am playing a character who frequently has a pool of temporary HP. Normally when people take damage you can see how they were to how they are to get some idea of how a battle is progressing. But what about people who have temporary HP? How do they appear both before they have lost any, and while they are losing that temporary HP?
It is up to the narrative style of the GM and players. Here are some ideas:
If the source of the temporary hit points is magical, you could describe them as a visible field, or a barrier visible only when struck.
If the source of the temporary hit points is non-magical then it depends on how you describe normal hit points. Because I think of hit points as a character's physical and metal stamina (and minor injuries), I would describe non-magical temporary HP as vigor and enthusiasm. If a character is struck, but still has some temporary hit points left over, I would probably describe how they defiantly block the blow, and, completely unfazed, raise their sword for the counterattack (or whatever). This is in contrast to losing actual HP: the character brings their shield up just in time, but they're jarred by the blow, and afterwards they look tired, ragged, and bruised.
ADDENDUM NOTE: In my earlier answer, I did not see, as Shadow Kras mentioned, that the question was based on Pathfinder's system. I had answered based on D&D 5e rules. I went to read Pathfinder's rules online and have changed my answer accordingly.
Still a Great Question!
In reading the Pathfinder rules for Temporary Hit Points...
"Certain effects give a character temporary hit points. These hit points are in addition to the character’s current hit point total and any damage taken by the character is subtracted from these hit points first. Any damage in excess of a character’s temporary hit points is applied to his current hit points as normal. If the effect that grants the temporary hit points ends or is dispelled, any remaining temporary hit points go away. The damage they sustained is not transferred to the character’s current hit points.
When temporary hit points are lost, they cannot be restored as real hit points can be, even by magic." (Source: https://www.d20pfsrd.com)
Also, after reading what Pathfinder said about "Injury and Death" and stabilizing a character, I would adjust my answer to fit Pathfinder this way...
1) How do they appear before they have lost temporary HP? Since Temporary HP is simply an "invisible barrier" around the character, if you will, that absorbs damage FIRST, until that character receives actual damage to their current or "real" HP, then they would not look any different than they did just prior to receiving the temporary HP.
If they were almost dead then they would still appear in the same condition. If they looked completely strong and uninjured, they would still appear the same. The Temporary HP would simply protect them from receiving damage to their current/real HP total until the Temporary HP were diminished in full. It does not change how they look UNLESS YOU DECIDE IT WILL. For Example: When they receive Temporary HP, a faint blue aura appears about them or the hair on their arms stands up or their eyes turn a violet color. It's up to you.
2) How do they appear while they are losing that Temporary HP? The same holds true. Again, nothing changes from how they looked just prior to receiving the Temporary HP UNLESS you decide you want them to look slightly different. Outside of that, their appearance will only change if and when they start taking damage to their current or "real" HP.
As far as describing what they are experiencing while taking damage and protected by Temporary HP, you can use something similar to the below...
Temporary HP from Magical Means: "As the orc slammed his ax into your chest, a faint rippling of warm energy washes across the area keeping the ax from biting into your armor and the flesh beneath."
Temporary HP from "Natural" Means: "As the Gargoyle's claws slashed across your chest, your natural defenses or rather unnatural defenses saved you once again, keeping them from ripping through your armor and int your flesh beneath."
Hope that is a better answer for your game system than what I provided initially for D&D. And actually the above would still apply to D&D as well.