Along with the actors who performed the part, special effects and costuming also contributed to the overall effect and it appears that Wilder put a lot of thought not only into his role but also into his wardrobe.
Dangerous Minds recently posted a letter Wilder wrote to the movie's director, Mel Stuart, concerning recent costume sketches and the actor's analysis is telling:
Much like with Andy Warhol's MoMA rejection letter, none of this should come as such a surprise but somehow, with the passage of time, it seems to shed new light on a beloved film or create an "aha" element. Just as we enjoy looking at "behind-the-scenes" photos depicting how movies create special effects, so too is it entertaining to go inside an actor's head and thought processes, particularly as it pertains to an iconic role.
I don't think of Willy as an eccentric who holds on to his 1912 Dandy's Sunday suit and wears it in 1970, but rather as just an eccentric -- where there's no telling what he'll do or where he ever found his get-up -- except that it strangely fits him: Part of this world, part of another. A vain man who knows colors that suit him, yet, with all the oddity, has strangely good taste. Something mysterious, yet undefined.
Read the entire letter, which comes from Willy Wonka & the Chocolate Factory (40th Anniversary Collector's Edition) over at Dangerous Minds.