disneytrivia:

Dick Van Dyke had his heart set on playing Mr. Dawes, Sr. in Mary Poppins, and said they didn’t have to pay him anything extra, he just really wanted to do it for the fun. Although Walt Disney had offered him the part of Bert right out, he made him audition for the part of Mr. Dawes, Sr.   

While viewing a make-up test for Van Dyke in the projection room, Disney saw him entertaining crew members on the test film between takes with some comic routines, among them the “stepping down” routine of an old man trying to step off a curb without hurting himself. The test film not only convinced Disney to cast Dick Van Dyke as Mr. Dawes, Sr., but Walt specifically requested that crew members “build a six-inch riser on the board room set so Dick can do that stepping-down routine” in the film.