[Easy E]

Let's talk about the role of a Confessor in the Ecclisarchy. 

What do they do and how do they do it?

Rampant speculation and heresy is welcome.    

[Kaled]

According to Codex Imperialis, an especially zealous preacher may be invited by a cardinal to become a Preacher.  The Confessor is not tied to a particular shrine, but is free to wander within the diocese and preach amongst the population.  They are the most extreme evangelical zealots, and are described as 'supreme architects of mass emotion' - under their spell, entire crowds will rush forwards to confess their personal heresies, denounce their neighbours as witches and reveal their relations as mutants and deviants.

[Charax]

They perform vile acts and then confess to them. If you don't follow the Creed, a confessor will come after you and add you to his list of foul deeds.

Only joking. Confessors are primarily orators, inciting hatred against heretics and traitors, and renewing the zeal of the Ecclesiarchy's followers. While many do this through their fiery oration and speeches to the masses, some act more as advisors and manipulators to people in power, inciting action on the Church's behalf through political means

[greenstuff_gav]

Prayer may cleanse the soul, but pain cleanses the body. Both are necessary for the survival of humanity -Confessor Ganinimus

Confessors are fiery individuals whose powerful oratories can stir a populace to rebel against a heretic lord or persuade an army to lay down its arms and surrender to the mercy of The Emperor.
...
confessors act as advisors to Imperial Guard colonels, Planetary Governots and sometimes, with special Ecclesiarchal dispensation, they even lead Wars Of Faith

-40k2 Codex Sisters Of Battle

it's worth noting that Confessors are a standard character, Redemptor Kyrinov is an Arch-confessor.. is there mention elsewhere about Confessor ranks?

[Easy E]

The only ranks I have seen is that Preists and Confessors are not the same thing.  Confessors are obviously of a higher order. 

However, I seem to recall the old black dex from 2nd Edition for Imperial Agents mentioned a few levels of Confessors.  Sadly, I no longer have the documents. 

[precinctomega]

All Imperial priests are confined to a parish.  This may be a geographical area or particular temple, or it may be a Rogue Trader vessel, an Imperial Guard regiment, pilgrim route or Inquisitor's staff.

Confessors are priests who have been released from their confinement to become free agents.  They may be able to whip up their own crusades, form their own fleets or simply wander from world to world as itinerant preachers.

Generally speaking, it would seem logical to assume that a Cardinal would appoint a Confessor with a specific purpose in mind: a Confessor may be charged with taking a particular interpretation of the Imperial faith to worlds that are perceived to be straying, or which cleave to an archaic or borderline-heretical version.  They may be appointed to found new dioceses in areas reconquered by the Imperium.  Or they may just be a way of getting rid of a troublesome and too-influential priest.

Think of Confessors as the spiritual equivalent to a Rogue Trader.

R.

[GhouraAgur]

Think of Confessors as the spiritual equivalent to a Rogue Trader.

R.

Where, then, do the Missionaries fit in, especially, the Missionarius Galaxia?

[mypunkrockisbetterthanyou]

Simply enough, missionaries may just as well go around spreading the word. People can't confess if they don't believe in the thing to which they can, nay, must, confess.

Regards,
 Rick

[precinctomega]

Missionaries aren't free-roaming.  They are normal priests with a parish.  It just happens that their parish won't sit within an established diocese.  A successful missionary is likely to become the cardinal of a new diocese.

R.