If you haven't shot the 15 before, try not to frame people near the edges. They grow thick and squat, like Gimli in the Lord of the Rings film. (Unless you are shooting someone whom you want to portray as marginal, thick and squat.) That is, the edges distort. All planes that are not flat also distort, so you idea about changing your level (e.g., kneeling), presumably to keep the film plane flat, is the best way to prevent unwanted distortions.
On the other hand, the 15 can be good for physiological (=psychological) exaggerations/deformings. I made a (21mm) digital portrait of a homeless advocate/vet, a powerful personality, who was shielding his eyes with one hand in regular perspective, but his other hand, resting on a chair arm close to me, was distorted in a way that conveyed power at rest.