Ok, when you fail a corruption check, you get some corruption tokens. The player gets to know that.

But my question is, do the characters know they just gained one (or more) corruption point? How should that be roleplayed?

And if the player knows his characters is about to gain a mutation, just needs one corruption point… does the character also feels he's about to be changed by the Ruinous Powers? Should it be considered metagaming if the wizard starts behaving too conservative (e.g. stops casting spells) because the player knows his character is about to draw a mutation?

I'd say it's up to the player really. Corruption is mostly a game mechanic and it's natural for a player to try to avoid getting more if they allready have a bunch, same as they might not be as reckless if they have a bunch of fatigue or stress.

Wether the character would actually feel the corruption… that's also pretty much up to the player. Maybe he notices that he's not feeling too good after handling that piece of warpstone. Or maybe he doesn't suspect a thing until he one morning wakes up with an extra arm.

By the way, I'd advise you to not give characters mutations, but rather cash in the corruption for challange dice if the player really doesn't want a mutation. The character easily becomes next to unplayable. It's your call though. It can be quite amusing to give a character a mutation and see how he handles it.

Corruption is just one of the many things I love about the game in terms of how it fuses in interesting mechanics that SUPPORT role-playing, a concept I constantly bash my head against in other game.

Its an option for the player, a kind of role-play que to grab on to, or ignore, though generally I reward heavily anytime players take the intiative and take these ques.

Some of the more creative ways my players have used corruption for are

1. Their appearance. Suddenly they start describing dark circles around their eyes, a change of hairstyle to a more, darker, sleeker look. Changing their close, all the sudden they have an urge to wear black or red. Stuff like that.

2. Anger, suddenly they start role-playing with a harsher edge, less patience, more aggression.

3. Often Im asked "where does the corruption come from", as it what chaos god does the corruption influence manifest, at which point they read the god and start displaying some of the behaviors of that god.

But ya role-playing is always optional in every regard so these are just mechanics to help you enhance the visuals of the narrative and the best way to get players to do it is reward heavily those that do.

Interesting. Suggesting minor changes to their bodies or behaviour as they accrue corruption (before mutating) seems a good idea to describe how it is manifesting on their characters. That will give a warning of what's going on and still remain in-character.