Earlier versions of the storyline, drafted in 1943 during the war, had the two cats appear as a sinister pair, suggesting the yellow peril. Originally named Nip and Tuck, they came to the house with Aunt Sarah while Jim Dear and Darling were still at home and made Lady look bad by committing a series of crimes such as stealing the baby's milk and robbing the icebox, then pinning them on her. In the novelization of the film by Ward Greene, written during its production, they tearfully expressed remorse over causing Tramp's impending execution by hiding the rat's body as a joke, and then tried to make amends.
Si and Am are a sinister pair of cats who use the love they gain from Aunt Sarah to get others into a large amount of trouble. The cats are sneaky and love to cause trouble and make messes for their own amusement. It is indicated that they care for nothing but each other and take advantage of Aunt Sarah, seeing her as a protector and not as the beloved owner she is.
Si and Am are identical seal-point Siamese cats. They are sleek and elegant cream-colored felines with angular, wedge-shaped faces, and large, blue almond eyes, which are oval-shaped and narrow out to points near the ends. They have a deep brown facial point with a darker, almost black muzzle, and dark triangular ears. Their noses are black and small, and they both have two middle incisors that hang over their lower lip; these buck teeth are a commonly used Asian stereotype during the late 1940s and 1950s. Si and Am also have dark brown "stockings" on their front legs and shorter "socks" on their hind legs and have dark, long, winding tails.
In the film, Si and Am's owner Aunt Sarah is in charge of Lady and the newborn child. The two cats are commonly seen toted in a picnic basket owned by Aunt Sarah, from which they emerge after her brief departure.
While Aunt Sarah is upstairs, Si and Am perform their trademark musical number, "The Siamese Cat Song". During their song, they wreak havoc throughout the household, claiming it as their own, and even attempt to eat both the family fish and family bird. During the closure of their song, Lady ferociously chases after them upon hearing of their intentions to sample some of the newborn baby's milk. Collateral damage ensues, and although the cats caused all the trouble, Si and Am feign injury in order to get away with it and lay all the blame on Lady. Si and Am are carried upstairs by Aunt Sarah as they deviously shake their tails. As a result, Lady is muzzled by Aunt Sarah. They are not seen for the rest of the film.
Si and Am reappear in the sequel to the film. They are first seen in the opening of the film while Jim Dear and Darling are taking a stroll in town, where they meet up with Aunt Sarah. She claims she will be attending their family picnic, as Si and Am love Independence Day. They later appear in Scamp's fantasy as he wishes to be a brave enough dog to take on threats like Si and Am.
Their final scene is in the middle of the film, where they attend a picnic with their owner. During the picnic, they are seen attempting to steal the turkey until Scamp arrives and scares them into Aunt Sarah's arms. They are not seen for the remainder of the film.
They make a notable appearance in Mickey's House of Villains during the villains' hostile takeover of the club. They are seen singing a part in "It's Our House Now!", and later on witnessing Mickey Mouse's climactic battle with Jafar. In the end, they flee with the other villains, after Jafar is sucked back into the lamp and defeated.
In Fantasmic!, Si and Am appear in the floating bubble musical sequence.
- These two characters have been cited as Asian stereotypes because of their squinty eyes and bucked teeth (or in their cases, fangs). These features have been altered in the sequel and House of Villains.
- With their names combined, "Siam" is the former name of the Kingdom of Thailand, referencing the cats' heritage and the name of the country.
- Because they're from a Thai breed, they speak with an Asian accent.
- Originally, they were to have been animated by Ward Kimball, but Walt Disney decided it was too fluidly animated and as a result, almost all his animation except one or two shots according to Kimball, were cut from the final film.
- They are the only Lady and the Tramp antagonists to be constantly involved in Disney Villain merchandise, without mentioning Aunt Sarah or The Rat.