  Casimir forces in a critical media are produced by spatial suppression of
order parameter fluctuations. In this paper we address the question how
fluctuations of a critical media relates the magnitude of critical Casimir
interactions. Namely, for the Ising model we express the potential of critical
Casimir interactions in terms of Fortuin-Kasteleyn site-bond correlated
percolation clusters. These clusters are quantitative representation of
fluctuations in the media. New Monte Carlo method for the computation of the
Casimir force potential which is based on this relation is proposed. We verify
this method by computation of Casimir interactions between two disks for 2D
Ising model. The new method is also applied to the investigation of
non-additivity of the critical Casimir potential. The non-additive contribution
to three-particles interaction is computed as a function of the temperature.
