Are the "Inspire" and "Terrify" special uses of the Command Skill meant to only be used on NPCs or could a PC also use them on his Warband? What do you think?

There's no reason not to use them on other player characters, provided they actually respect your IC authority, be it personal or otherwise.

I agree with DBG.

The ruleset unfortunately does not make much of a distinction between the personal dispositions of the player characters, but there's no reason a good GM could not interpret an effect by the spirit rather than the letter of the rules.

Of course, the opposite would also be true. For example, a player character who, for whatever reason, is not very spiritual may only receive a diminished bonus - or even none at all - from a Missionary's Litany of Hate.

I believe these are meant to be used on PCs. Command is the 'combat skill' for social characters, and an important buffer against powerful fear effects.

Hmmh, after re-reading both books, DH1 explicitly mentioned NPCs, but in DH2 it only says "followers" which could easily be interpreted as including player characters subordinate to whoever makes the Test. With "Inspire", they even added a special use from the DH1 Inquisitor's Handbook, where a successful Test provides a Bonus to the follower's next Skill or Characteristic Test, so it's all in the rules this time.

Although it does seem pretty e-z mode compared to how it worked in DH1, where the bonus was only +5 instead of +10, and it actually worked only on followers who were already "Devoted" or "Fanatical" in their Disposition (though really, I think the most sensible option would have been to just use Skill Test modifiers representing the targets' Dispositions).

Hmmh, after re-reading both books, DH1 explicitly mentioned NPCs, but in DH2 it only says "followers" which could easily be interpreted as including player characters subordinate to whoever makes the Test. With "Inspire", they even added a special use from the DH1 Inquisitor's Handbook, where a successful Test provides a Bonus to the follower's next Skill or Characteristic Test, so it's all in the rules this time.

Although it does seem pretty e-z mode compared to how it worked in DH1, where the bonus was only +5 instead of +10, and it actually worked only on followers who were already "Devoted" or "Fanatical" in their Disposition (though really, I think the most sensible option would have been to just use Skill Test modifiers representing the targets' Dispositions).

I suspect some of the details about the specific relation between the Commande- (skill use-)r and the commandees is to give narratively flexbility to the situation. I think the PCs can establish for themselves if they're following the orders of this other PC in X situation. Maybe they all follow the Warrior's lead during combat, but during investigation they follow the Seeker.

As for Inspire, I think I'd apply it more broadly than Terrify.