We provide a detailed analysis of the correlation length in the direction parallel to a line of modified coupling constants in the ferromagnetic Potts model on Z^d at temperatures T>T_c. We also describe how a line of weakened bonds pins the interface of the Potts model on Z^2 below its critical temperature. These results are obtained by extending the analysis by Friedli, Ioffe and Velenik from Bernoulli percolation to FK-percolation of arbitrary parameter q>1.