A Great 'Eureka' Moment....
For me, a great part of the beauty of Keaton's acting is that we see him thinking; what I call his "Eureka" or "Aha!" moment. One of the best in all his films is in "The Three Ages" (his absolute best is during the poker game in "Go West") when Keaton is contemplating how to win the chariot race, it's dazzling to watch "the wheels in motion" - to see that magnificent intellect light up his beautiful face. You just don't see that in many actors, if at all, and certainly not to the degree you do with Keaton.
I really don't know how anyone after watching Keaton on screen could have thought he wasn't smart! His in-laws must have been blind!