Observations of AGNs with different high energy instruments indicate on formation of gamma rays in compact regions (i.e., in blobs). This, in particular, is supported by the shapes of recorded lightcurves and by the ultra-fast variability time scales obtained with ground based Cherenkov detectors (e.g., with H.E.S.S.). However, the conventional approach for modelling of these sources features calculations of the non-thermal high-energy emission leaving the question related to the nature of these blobs less addressed. I will discuss a specific scenario for blobs in AGN jets: formation of blobs at interaction of AGN jet with a star. Despite the apparent generality, once applied to a specific source the scenario implies quite strict consequences on the properties of the jet and possible production mechanism at work. To illustrate this, I consider two exceptionally challenging cases: the ultra-fast variability detected with H.E.S.S. from the blazar PKS 2155-304, and extremely bright flare register with Fermi/LAT from 3C454.3. The scenario, if realised, can readily explain several puzzling features, like the lightcurve shapes and variability time scale, and allows to constraint the key properties of the sources. (to be updated)