"Push processing" generally makes grain more obvious and builds contrast. B&W stock and Ektachrome could be pushed, Ektachrome could be "hypersensitized" for astral photography using a pressurized tank and nitrogen. Quite a process. I've known a couple guys who did that, they had some amazing results. Both used Questar 'scopes and "clocks" for long exposures, as well.

With Kodachrome you were locked into the ISO/ASA of the emulsion.