  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 $\mathbb{Z}^d$ at temperatures $T>T_c$. We also describe how a line of
weakened bonds pins the interface of the Potts model on $\mathbb{Z}^2$ 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 $q>1$.
