The reason i think this (gif below) kiss was improvised is because

if you look at this next move where she goes in to hug josh peeta he sort of lingers and is not sure if she was gonna kiss or hug him look what i mean he keeps his head there for a split second and sort of turns his head (gif below) 

if he was not sure that that was going to be a kiss or hug he had an idea in his head that it could be either and shes done it before ie. 5 seconds ago.

(all creds go to  for the gifs)