An interpolation theorem holds for many standard modal logics, but first order $S5$ is a prominent example of a logic for which it fails. In this paper it is shown that a first order $S5$ interpolation theorem can be proved provided the logic is extended to contain propositional quantifiers. A proper statement of the result involves some subtleties, but this is the essence of it.
"Interpolation for first order $S5$." J. Symbolic Logic 67 (2) 621 - 634, June 2002. https://doi.org/10.2178/jsl/1190150101