Sylvester's trademark was his sloppy, stridulating lisp. In his autobiography, That's Not All Folks!, voice actor Mel Blanc stated that Sylvester's voice is based on that of Daffy Duck, plus the even-more-slobbery lisp, and minus the post-production speed-up that was done with Daffy's. Conventional wisdom is that Daffy's lisp, and hence also Sylvester's, were based on producer Leon Schlesinger's. However, Blanc made no such claim. He said that marire penis Daffy's lisp was based on him having a long beak, and that he borrowed the voice for Sylvester. He also pointed out that, minus the lisp, Sylvester's voice was fairly close to his own (a claim that his son Noel Blanc has confirmed). In addition, director Bob Clampett, in a 1970 Funnyworld interview, agreed with Blanc's account concerning Schlesinger.
To emphasize the lisp, as with Daffy's catchphrase "You're desthpicable", Sylvester's trademark exclamation is "Sufferin’ succotash!", which is said to be a minced oath/euphemism of "Suffering Savior".
Prior to Sylvester's appearance in the cartoons, Blanc voiced a character named Sylvester on The Judy Canova Show using the voice that would eventually become associated with the cat.
Sylvester is a tuxedo cat who shows much pride in himself, and never gives up. Despite (or perhaps because of) his pride and persistence, Sylvester was, with rare exceptions, placed squarely on the "loser" side of the Looney Tunes winner/loser hierarchy. His character was basically that of Wile E. Coyote while he was chasing mice or birds. (One cartoon episode The Wild Chase paired Sylvester and Wile E. Coyote against the Road Runner and Speedy Gonzales. In the end both Sylvester and Wile E. fail as usual.) He shows a different character when paired with Porky Pig in explorations of spooky places, in which he doesn't speak as a scaredy cat. (In these cartoons, he basically plays the terrified Costello to Porky's oblivious Abbott.) He also appears in a handful of cartoons with Elmer Fudd, most notably in a series of cartoons underwritten by the Alfred P. Sloan Foundation extolling the American economic system.
Perhaps Sylvester's most developed role is in a series of Robert McKimson-directed shorts, in which the character is a hapless mouse-catching instructor to his dubious son, Sylvester Junior, with the "mouse" being a powerful baby kangaroo which he constantly mistakes for a "king-size mouse". His alternately confident and bewildered episodes bring his son to shame, while Sylvester himself is reduced to nervous breakdowns.
Sylvester also had atypical roles in a few cartoons:
• Kitty Kornered (1946), a Bob Clampett cartoon in which a black-nosed, yellow-eyed Sylvester was teamed with three other cats to oust homeowner Porky Pig.
• Back Alley Op-Roar (1948), a Friz Freleng cartoon (actually a remake of the 1941 short Notes To You) wherein Sylvester pesters the sleep-deprived Elmer Fudd by performing several amazing musical numbers in the alley (and even a sweet lullaby ("go to sleep...go to sleep...close your big bloodshot eyes...") to temporarily ease Elmer back to the dream world ... if only temporarily.
• The Scarlet Pumpernickel (1950), a Chuck Jones cartoon in which Sylvester plays the Basil Rathbone-like villain to Daffy Duck's Errol Flynn-esque hero.
In the 1970s and 1980s, Sylvester appeared in various Warner Bros. television specials, and remaining in the 1980s, he appeared in the feature-film compilations.
In the television series Tiny Toon Adventures, Sylvester appeared as the mentor of Furrball. The character also starred in The Sylvester and Tweety Mysteries. In the series, he plays the narrator in the beginning of episodes.
In Loonatics Unleashed Sylvester's descendent is Sylth Vester, a hitman hired by the villain, Queen Granicus to kill the Royal Tweetums so she won't have to lose her throne. Despite his best efforts he's beaten by the Loonatics. Later on the series, it is shown that he's not entirely a bad guy, for he helped the Loonatics finding the Royal Tweetums (who was hidden) and fighting against Optimatus and Deuce's, and their plan to take over the Universe. Just like his ancestor, Sylth Vester tries to kill Tweety's descendant, using all kinds of tricks.
In 1985, Sylvester could be heard in an episode of the game show Press Your Luck. Host Peter Tomarken had earlier incorrectly credited his catchphrase "Suffering Succotash!" to Daffy Duck. Even though all three contestants had correctly answered "Sylvester," they were ruled incorrect. In a segment produced later and edited into the broadcast, Sylvester phoned Tomarken and told him, "Daffy Duck steals from me all the time." All three participants returned to compete in future episodes.
Sylvester has died the most out of any Looney Tunes character having died in "I Taw a Putty Tat", "Back Alley Op-Roar", "Peck Up Your Troubles", "Satan's Waitin'", "Mouse Mazurka", and "Tweet and Lovely".
Western Publications produced a comic book about Tweety and Sylvester entitled Tweety and Sylvester first in Dell Comics Four Color series #406, 489, and 524, then in their own title from Dell Comics, then later from Gold Key Comics (#1-102, 1963-72). In a Garfield cartoon, he made a cameo by sending Rosalina (Garfield) a love letter.