David Tennant's improv in Harry Potter and the Goblet of Fire changed the movie's Mad-Eye Moody twist. The fourth Harry Potter movie introduced several major new characters to the franchise as Hogwarts—in addition to students from the Durmstrang Institute and Beauxbatons Academy of Magic—competed in a magical tournament. This included the first proper appearance of Voldemort (Ralph Fiennes) and the debut of Robert Pattinson's Cedric Diggory. Two of the new faces who played an integral part in Goblet of Fire's story, though, were Barty Crouch Jr (David Tennant) and Alastor Mad-Eye Moody (Brendan Gleeson).
Moody played a pivotal role in the movie as he helped Harry Potter throughout the Triwizard Tournament. However, Harry and everyone else at Hogwarts was not aware that Mad-Eye Moody was an imposter. Harry Potter and the Goblet of Fire eventually revealed the twist that the seemingly cruel, albeit helpful teacher was actually Barty Crouch Jr. in disguise. The follower of Voldemort subdued Moody to take his place using Polyjuice Potion, and ensured that Harry Potter would be selected from the Goblet of Fire. While this twist also happened in J.K. Rowling's book, the movie adaptation included a big change that could've clued unsuspecting viewers into what was happening.
When David Tennant began playing Barty Crouch Jr., he was given the freedom as an actor to make the wizard his own. This included Tennant improving a few unique tics to his Harry Potter character. The most notable of these unique habits is Crouch repeatedly licking his lips like a snake. Neither Harry Potter and the Goblet of Fire's source material or script called for Tennant to have this trait, but director Mike Newell liked the improvisation. Since Brendan Gleeson then had to play Mad-Eye Moody as Barty Crouch Jr., the actor then adopted this change and included a few lip licks of his own.
While Harry Potter and the Goblet of Fire was always going to include the Crouch Jr./Mad-Eye twist, Tennant improvising Barty's lip-licking habit brought a new aspect of the reveal to the screen. The most prominent example of Mad-Eye Moody having Crouch's licking tic came immediately after the second task of the Triwizard Tournament. It even happened as Moody spoke to Crouch Sr., who immediately recognized the tic of his son. However, Crouch Sr. wasn't able to divulge this information to others as he died shortly after encountering his son in disguise.
Those familiar with J.K. Rowling's Harry Potter books might have always known that the Mad-Eye Moody twist was coming, but David Tennant's improv gave the movie a way to tease it to observant viewers. Tennant's scenes as Barty Crouch Jr. featured plenty of moments where he licked his lips. Those who caught on to this habit and then noticed Moody do the same thing might have been able to see Harry Potter and the Goblet of Fire's big twist coming.