@Speravir Thanks :). However, I would point out that IMHO the solution given with callouts does not provide interactivity: the user is forced to provide overlay specifications thus the callout will appear just in that moments. But you suggested to me an idea: I'll add it to Explanatory bubbles in beamer
@mabartibin (hope you get this) Curious behaviour with pgf-blur with the latest PGF. It seems that fadings might have to have unique names: when compiling the example then I found that all the drop shadows were the same shape (that of the first shadow). Putting in a counter to make the names unique fixed this.