The main thing is.. How do space marines act? THey're zealous and they yell alot and get excited to fight chaos etc. They're extremely passionate about what they do.

Guardsmen are more human so.. put yourself in a well trained warriors position staring down death that comes in the form of something that looks like it jumped straight out of hell (cause it pretty much did.)

Inquisitors are usually eerily calm even in the face of extreme adversity and usually quite zealous though they rarely show it.