How much information should I reveal in a postdoc interview, when I am also applying for a fellowship at a different institution? I have been invited for an interview to a post-doc position. In the same time, I am applying for a post-doc fellowship to a different organisation. If that fellowship application will be successfull, I will have to quit the postdoc position (where I am going to do interview) somewhere in the middle, because I am more interested in that position with the fellowship. 

I know, I don´t have any clues whether any of the two positions will work out, but how should I address this fellowship opportunity on the intreview?

Should I let them know in advance that I am applying for a fellowship and let them decide whether they want to interview me anyway? Or should I mention only at the interview, that I am applying for a fellowship? Or shouldn´t I mention it at all?
They will probably ask you if you are applying or evaluating other positions during the interview, if they don't you don't have to bring it up.

But if they do ask, just say you currently don't have any offer in hand but that you are applying to other positions. When and if they make you an offer, at that point you should decide to accept it or not, or to ask for extra time to make your decision. But switching post-doc half way will not be good for your CV or your research, but that's another question.