High-resolution intracavity dye laser spectroscopy has been used to obtain sub-Doppler spectra of transitions to 350 rotational levels in the 4(0)(1) band of the ($) over tilde A (1)A(2)-($) over tilde X (1)A(1) electronic transition of thioformaldehyde. Ground state combination differences from the sub-Doppler spectra, combined with microwave and infrared data, have been used to improve the ground state rotational and centrifugal distortion constants of H2CS. The upper state shows a remarkable number of perturbations. The largest of these are caused by nearby triplet levels, with matrix elements of 0.05-0.15 cm(-1). A particularly clear singlet-triplet avoided crossing in K-a’ = 7 has been shown to be caused by interaction with the F-1 component of the 3(1)6(2) vibrational level of the ($) over tilde a (3)A(2) state. At least 53% of the S-1 levels show evidence of very small perturbations by high rovibronic levels of the ground state. The number of such perturbations is small at low J, but increases rapidly beyond J=5 such that 40%-80% of the observed S-1 levels of any given J are perturbed by ground state levels. Model calculations show that the density and J dependence of the number of perturbed levels can be explained if there is extensive rotation-induced mixing of the vibrational levels in the ground state.