  Maintaining the preserved supersymmetry helps to find the effective
Lagrangian on the BPS background in gauge theories with eight supercharges. As
concrete examples, we take 1/2 BPS domain walls. The Lagrangian is given in
terms of the superfields with manifest four preserved supercharges and is
expanded in powers of the slow-movement parameter lambda. The O(lambda^0) gives
the superfield form of the BPS equations, whereas all the fluctuation fields
follow at O(lambda^1). The effective Lagrangian is given by the density of the
K"ahler potential which emerges automatically from the lambda expansion making
four preserved supercharges manifest. More complete account of our method and
applications is given in hep-th/0602289 in which the case of non-Abelian
vortices is also worked out.
