A method for determining a set of modifiable elements in a group of pictures of a coded bit-stream representative of an audio video content is disclosed. The method comprises determining a set of a candidate modifiable elements wherein a candidate modifiable element comprises a modified value of the coded bit-stream and a spatial propagation map associated with the modified value, a spatial propagation map comprising pixels whose decoding is impacted when the modified value is placed in the coded bit stream; determining a heat map for each reference frame, the heat map comprising, for each pixel of the reference frame, an information representative of the usage of said pixel for temporal prediction during the decoding of said part of the coded bit-stream coding a group of pictures; obtaining a set of modifiable elements among candidate modifiable elements, a modifiable element having a spatial propagation map that does not overlap with corresponding heat map.