We propose a new method for the determination of depth formation of absorption lines in spectra of late-type stars and substellar objects. Specifically, we estimate the contribution of any level of stellar atmosphere to the equivalent width of an absorption line. The method can be used for the case of the formation of atomic lines on the molecular band background. We consider the cases of the formation of the lithium resonance doublet lines in the atmospheres of the Sun, red giant (M2 III) from the of RS Oph system, C-giants WZ Cas (C6) and brown dwarf LP944-20 (M9 V).