Now, how does one do this and still let the film run normally. In other words, how do I concentrate this special effect only on a single persons face? Just push me into the right direction so I can aquire the know-how :)
(That is going to look quite wicked, if not scary...)
The key here is Face Tracker. You can attach other facial elements over the footage once you've tracked the features.
The next thing is figuring out threshold value at which a feature changes (such as an eye blinking or mouth movements and expressions) and assigning different images based on it, which will be a matter of expressions.
The other option is to always use the same overlay features but use the Puppet tool to animate them based on the face tracker points.
(I certainly hope you're not tryng to convey positive emotions, for this approach will have a Frankenstein's monster feel.)