The behavior of an individual particle interacting with phonons in a many-boson system has been investigated in the scheme of the collective description by using the number density operator and the phase operator as the collective variables. For a model-interaction potential which is strong but of long-range, it has been shown that the third order anharmonic terms to appear in this collective description give rise to the backflow effect of the individual particle which has been discarded in the random phase approximation. The Feynman-Cohen wave function and the dipole-type interaction between individual particles transmitted by phonons have been obtained. In relation to Puff's theory of sum rules the spectral function associated with the density-density correlation function for individual particles has also been investigated.