Know-It-All is one of the children riding the Polar Express to the North Pole. When Hero Boy first gets on, Know-It-All asks what kind of train the Polar Express is, leaving Hero Boy confused. However, Hero Girl joins in and simply says that it is a magic train that goes to the North Pole. Know-It-All then goes on to give a detailed explanation of the train before being cut off after seeing Herplosheimer's outside the window and calls out to the other children to come and see. When he sees all the presents in the display, he claims that he wants all of them.
Later, the Conductor comes by to punch everyone's tickets. He punches "LE" out of Know-It-All's ticket, which Hero Girl would also get later in the film. Know-It-All tells Hero Boy that he thinks the Conductor is showing off his ticket punching skills. He is also surprised that the train is stopping at Billy's house, thinking Hero Boy was the last pick up. When Billy was running after the train, Know-It-All suggests to Hero Boy that he pulls the emergency brake, but ironically, tells him on the Conductor afterwards.
Know-It-All is also served hot chocolate along with all the other children. When Hero Girl goes to give a cup to Billy, Know-It-All tries to persuade her out of it due to it being too dangerous for children to cross between moving cars without an adult. The Conductor then shows up to assist her. When Hero Boy goes to give her ticket back, he warns him about the rule again. After the Conductor realizes he forgot to punch Hero Girl's ticket and takes her away after finding out she lost it, Know-It-All thinks that she is going to get thrown off the train and that the train will slow down for her, but not stop. This gives Hero Boy the idea to stop the train for her using the emergency brake again, but stops after finding her ticket stuck in the air vent, grabs it and goes after Hero Girl and the Conductor.
Know-It-All does not appear again until Hero Boy, Hero Girl and the Conductor get back to the coaches after the train went down Glacier Gulch and across the Ice Lake. He tries telling Hero Boy about what happened, thinking he missed everything, and claims that an optical illusion made it only appear as though it was on the ice, but Hero Boy does not listen and instead goes to the observation car to watch Billy and Hero Girl sing "When Christmas Comes to Town."
At the North Pole, while Hero Boy, Hero Girl and Billy were in the big bag of presents, something pulls on Billy's leg, so Hero Boy and Hero Girl help pull him free. It turns out to have been Know-It-All, who claims he was in the bag to find out what he is getting for Christmas, but all he found was some underwear. When the children get rescued from the bag, Know-It-All tells the elves that he was just following the others. Later, when Santa Claus arrives, Know-It-All impatiently asks if he will receive the first gift of Christmas, but Santa tells him to show some more patience and a little bit of humility. Know-It-All also watches Hero Boy being given the first gift and Santa Claus taking off to deliver the presents.
After that, the Conductor and all the children head back to the train for the return journey. The Conductor punches out "ARN" out of Know-It-All's ticket, resulting in "LEARN," though he initially reads it as "LEAN" due to one of his fingers covering the R. The Conductor tries explaining the mistake, but that only makes Know-It-All angry until he looks at his ticket again and realizes his mistake. He apologizes for his outburst and gets on the train. He is also one of the children who wants to see Hero Boy's silver bell and feels sorry for him when he finds out he lost it.
When the train stops to let Billy off, Know-It-All questions where he is going before Billy tells him the train stopped at his home, so Know-It-All wishes him a Merry Christmas. He also says goodbye to Hero Boy when he is being dropped off, still feeling sorry about the bell. He points out, however, that, "it's the thought that counts."
Know-It-All is one of many characters from the film to not appear in the book.
In the film, Know-It-All describes the locomotive of the Polar Express to be a Baldwin 2-8-4 S3 class virtue type steam locomotive and said it was built in 1931 at the Baldwin Locomotive Works and weighted 456,100 pounds. In reality, Baldwin did built 35 locomotives of the S3 class for the Erie Railroad, except they weighed 457,000 pounds and were built in 1928. Meanwhile, the locmotive in the film is based on Pere Marquette 1225, which is a member of the N1 class that were built at the Lima Locomotive Works in 1941 and weighed 442,500 pounds.