Three new songs were written for the film by Jerome Kern and Oscar Hammerstein II, the show's original creators. There is also, instead of the Chicago World's Fair scene which begins Act II in the stage version, a new sequence that occurs right after the third new song, in which Magnolia's baby is about to be born and there is no doctor to be found. Meanwhile, Ravenal (Allan Jones) is away gambling rather than at his wife's bedside, and the boat's deckhand Joe (Paul Robeson) grabs a raincoat, goes in search of a doctor, finds him, and rows him back to the show boat - all this during a fierce storm.
There is also a new conversation between Cap'n Andy and Parthy shortly after Magnolia and Ravenal have moved to Chicago that is not in the play. In the conversation, we find out that Ravenal and Magnolia have moved to a smaller residence after living in a fancy hotel, but we are left to find out why later on.
There are a few other transitions that have been added to the film which were not in the original show. Scene 3 of the original 1927 stage production took place outside a saloon and originally featured two musical numbers - "Life Upon the Wicked Stage" and "Till Good Luck Comes My Way"; these are heard instrumentally in this film later although they aren't sung, but much of the dialogue of this scene (which in the film takes place inside the saloon) is retained. The new transitions include a couple of moments written by Oscar Hammerstein II for the film - one in which we hear the conversation of the crowd filing out of the boat's auditorium after a show has ended, and one which shows them filing in while a child not allowed to see the show cries for his mother. (Around his neck is a misspelled lettered sign saying "In kase [sic] I get lost, my parents are...etc.")
A new conversation, potentially considered offensive by modern standards, was written for Joe and Queenie (Hattie McDaniel), who are black. It takes place just after Joe first appears, before the lead-in dialogue to the song "Ol' Man River". Queenie berates Joe for being a "slowpoke" and not bringing the flour that she needs for baking the bread fast enough; she calls him a "lazy good-for nothing", which Joe takes very good-humoredly. Queenie then takes the flour, calls Joe "the laziest thing that ever lived on this river'' (to which Joe replies mysteriously "That's sayin' a whole lot") . Queenie then goes back into the boat while Joe remains outside on the wharf. Then we return to the dialogue in the original stage version, and the conversation between Joe and Magnolia (Irene Dunne) that leads into "Ol' Man River". Some of Queenie and Joe's conversation was incorporated by Oscar Hammerstein II into Act Two, Scene Seven of the 1946 stage revival of "Show Boat", and it has remained in that scene ever since (except in the 1994 Harold Prince revival of the show).
The last sequence of the film (which covers the final ten minutes) is quite different from the one in the show because it takes place in New York rather than on the Mississippi River, although we still do get to hear Ravenal and Magnolia reprise "You Are Love" and Joe reprise "Ol' Man River" at the end. In the original show, we do not get to see Magnolia and Ravenal's daughter Kim (played as an adult in the 1936 film by Sunnie O'Dea and as a child by Marilyn Knowlden) make her debut as a leading lady on the Broadway stage as we do in the film. In the final scene of the stage version, we learn that Kim has already become a leading lady on Broadway and is returning to the show boat with her mother for a reunion secretly arranged by Cap'n Andy. When they arrive, Magnolia encounters Ravenal on the levee and the two are reconciled. In the 1936 film, the two are reconciled at the Broadway theatre where Ravenal has been working as a doorman and where Kim is making her starring debut.
Despite what sounds like extensive rewriting from the previous description, only about twenty minutes of the original show are actually changed. Otherwise, the storyline and even most of the dialogue of this film version is exactly that of the original show, and nearly all of the scenes retained from the show are sequenced in exactly the same order as they are in the stage version. The only scenes which have been switched around are the one in the convent school hallway, and the Trocadero rehearsal scene - onstage the rehearsal scene comes first. However, the dialogue and the singing for both scenes is still the same. As in the show, the story covers a period of roughly forty years, from the late nineteenth century to the early twentieth century. Aside from the three additional songs, Joe and Queenie's two new dialogue scenes, Cap'n Andy and Parthy's new conversation, and the new final sequence, the 1936 film follows the show nearly word-for-word, although edits have been made to the script to fit it all into a two-hour length, rather than three hours as on stage. All of the revisions and additions have been made by Oscar Hammerstein II, the show's original librettist, and not by some little-known screenwriter.
The film features a total of thirteen songs, not counting all the reprises and instrumental renditions of some of the numbers. The original show featured a total of nineteen different numbers, not counting the reprises. (Reprises are similar to encores, except that sometimes the lyrics of the song being reprised are changed to fit whatever situation is going on in the show at the moment.)