I think the fourth wall stuff works better here than with That Woman, and I love the part where Biddle catches John Lawless talking to the fourth wall. Aside from the score, it's little things like that that make the movie.
Such criticisms of Millionaire's loose story structure ring hollow in my ears, for I consider That Woman to be an even bigger mess. What is the point of Admiral Boom or Uncle Albert? Both characters could have been removed without harming the equally threadbare narrative. How can anyone buy the ending where Mr. Banks gets his job back after essentially killing his boss's father? And then there's That Woman herself. Even when toned down by Walt, the Shermans, and Walsh & DaGradi, she's needlessly cruel. She doesn't explain anything (if that isn't lazy storytelling, I don't know what is), she threatens the children, she's rather condescending to Bert (who, for some reason, is crazy about her; he must be a glutton for punishment), and she exposes the children to industrial pollution (Designing Women called them out on that one in 1990; bravo, Mary Jo!). And that's just for starters. And don't get me started on Mrs. Banks. On the whole, I agree with Michael Barrier when he said of the film in Walt Disney: An Illustrated Man:
Given the attention that Walt Disney himself lavished on [the film], it should have been his triumph, and it certainly was such as measured by the box office. Beneath its bright surface and cheerful songs, however, there was a lurking failure that was Disney's in his once strong role as story editor. [The film] had no story apart from the transformation of Mister Banks, the father of the children whom [that woman], the magical [sic] nanny, takes under her care; but David Tomlinson, who played the role, was a supporting actor, nothing more, too clearly confined by mannerisms and temperament to roles calling for a stuffy, easily ruffled Englishman. Disney wanted [that woman] herself at the center of the film, and so putting a strong actor—much less a difficult actor, someone like Rex Harrison—in Tomlinson's place was unthinkable; but without such an actor, the film could be only a succession of musical numbers held together by a very slim narrative thread. Mister Banks' transformation has no weight, a fact underlined by the very casual (and wholly unbelievable) manner in which he regains his job at a bank after he has been liberated by losing it. A centerpiece dance number on the rooftops has no strong dancer leading it; Dick Van Dyke hardly fulfills that role.
Everywhere that Disney's hand is most evident, as in some of the casting and incidental "business," [the film] suffers from debilitating weaknesses. At the least, a question mark hangs over the casting of Van Dyke as Bert the chimney sweep—the role would have benefited from an actor such as Tommy Steele, who, like Andrews, had roots in British music-hall comedy. Bob Thomas told how Disney "made a habit of 'walking through' the sets after they had been built, searching for ways to use them. Bill Walsh [the film's principal writer and co-producer] described a visit by Walt to the Bankses' living room in search of reaction to the firing of Admiral Boom's cannon: 'Walt got vibes off the props. As he walked around the set he said, 'How about having the vase fall off and the maid catches it with her toe?' or, 'Let's have the grand piano roll across the room and the mother catches it as she straightens the picture frame.'" But the havoc supposedly caused by Admiral Boom's cannon—on a regular schedule!—is simply ridiculous. There is no reason to believe it would be tolerated in a well-ordered London neighborhood. Here, as elsewhere, [the film] is the sort of shallow fantasy that undermines its own premises.
Tommy Steele as Bert? Hmmm, I'll let that simmer for awhile while I play "I'll Always Be Irish" on my iPod, but Van Dyke and Tomlinson were put to much better use in their other Sherman Brothers musicals.
Ironically, it was Bill Walsh's idea to turn Millionaire into a musical in the first place; it was a book and then a play, and IIRC it was no less episodic there than on film. Walt took Walsh off the picture and put him on Blackbeard's Ghost. Perhaps with a revamped book, the musical version could work on the stage.
The Shermans wanted Rex Harrison to play Biddle (with Gladys Cooper as his mother again, it would have been a My Fair Lady reunion), but he did Doctor Dolittle instead. Good call?