The short scene during the party where Ace pulls the cellist's arm as he walks by, making him produce a false tone, was improvised by Jim Carrey.
The voice of Ace Ventura and the manner in which he speaks was added by Jim Carrey only after several read throughs of the script. The voice was something Carrey used in his stand up routine. The "All righty then" was the catch phrase of one of his stand up characters and after the lines from the script weren't feeling right, he added it to the script and read through it again using that voice for all the lines. It was such an improvement that it became the main Ace Ventura personality trait.
The original concept was that the Pet Detective character was a bumbling idiot, until Jim Carrey was cast and his character was rewritten. Carrey reportedly had two conditions for doing the movie: the comedy had to be as zany as physically possible, and his character, despite his quirks and eccentricity, should be good at what he does.
Originally Ace wasn't written to be animal-related, but more of a generic Sherlock Holmes parody. Screenwriter Jack Bernstein came up with the pet detective idea after watching a David Letterman sketch about pet owners.
Ace Ventura compares Ray Finkle's room to Hannibal Lecter's prison cell from El silencio de los corderos (1991). Seventeen years later, in Jim Carrey (2011), Jim Carrey recounted that he asked Anthony Hopkins to dinner and discovered that his approach to Ace Ventura was the same as Hopkins' to Hannibal Lecter: they moved like animals. Carrey based Ventura's movement on birds and Hopkins based Lecter's movements on reptiles. Carrey confessed that Hopkins was becoming Hannibal Lecter right at the table.