  We present a systematic introduction to the diagrammatic method for practical
calculations in inflationary cosmology, based on Schwinger-Keldysh path
integral formalism. We show in particular that the diagrammatic rules can be
derived directly from a classical Lagrangian even in the presence of derivative
couplings. Furthermore, we use quasi-single-field inflation as an example to
show how this formalism, combined with the trick of mixed propagator, can
significantly simplify the calculation of some in-in correlation functions. The
resulting bispectrum includes the lighter scalar case ($m<3H/2$) that has been
previously studied, and the heavier scalar case ($m>3H/2$) that has not been
explicitly computed for this model. The latter provides a concrete example of
quantum primordial standard clocks, in which the clock signals can be
observably large.
