Woo! Finally a Disney question.

Walt Disney's involvement in feature films typically involved guiding the production in terms of introducing the initial ideas (e.g. wanting to make a classic story or folk tale into a movie), making basic business deals concerning the rights of work (although his brother Roy was still more of the business side of the company while Walt was the creative side), commissioning songs, and having authority for final editing decisions.

If you want to look at it bluntly, he would have a desire or an idea and get others to do the writing, storyboarding, animating, and songwriting.

If you have seen Saving Mr. Banks, the recent film depicting the events of the production of Mary Poppins, it does a decently good job illustrating Walt Disney's involvement. Note: the film isn't completely accurate, especially regarding the situation and reactions of Travers.

If you are looking for answers such as writing scripts, animating and lending his voice, that type of involvement was back in the 1920s and 1930s (and for some elements, the 1940s) for his cartoon shorts. By the 1940s, he got very busy especially because the Disney studio was transformed into a propaganda machine for the US govt (I in no way intend this description to be derogatory - it's simply descriptive of the company's involvement in the war) and in the 1950s, Disney was dealing with the construction of Disneyland (and then in the 1960s, the initial plans for Walt Disney World in Florida).