Rishi Kapoor plays Saif's father and sings a song while at the Paris airport. The song, "Mai Shyer To Nahin" (I am not a poet) is from his famous movie Bobby (1973), as are many other inside jokes in the movie.
When Saif was trying many different hair styles for this movie, one of the assistants said for one hairdo, "This looks like a Tom Cruise style". Director Kunal Kohli heard this and included it in the script. Saif thought that it was too childish, but later Kunal was able to convince him
Hum Tum is a fun-filled film and has several hidden references to Rishi Kapoor's yesteryear hit Bobby. Rishi plays Saif Ali Khan's father in the movie and is seen humming the song, 'Main shayar toh nahi.' Also when Rishi Kapoor meets Kirron Kher in the film, she tells him that he can call her Bobby. However the actor retorts saying that he would call her Dimple. It was Dimple Kapadia who played the title role of Bobby in the film.
Hum Tum is loosely based on Hollywood film When Harry Met Sally. Aditya Chopra had reportedly announced that he was planning remake a Hollywood film, starring Shah Rukh Khan and Kajol in the lead roles with Rani Mukerji in a supporting character. However, the outcome was completely different with Hum Tum being Saif Ali Khan's solo hit starring Rani in the lead role. The film was directed by Kunal Kohli.
Before Saif Ali Khan made it big in Bollywood with the success of Hum Tum, the film was offered to several other actors in the industry. According to IMDb, Vivek Oberoi was approached to play the role of Karan Kapoor but he decided against it. Also A-list stars like Aamir Khan and Hrithik Roshan were offered the roles too, but did not make it to the final cast. Also Kajol was supposed to be the leading lady, but was later replaced by Rani.
Hum Tum was a hit at the box office and made Saif Ali Khan a bankable name. However, the makers had different plans for the movie. Apparently, the makers of Hum Tum had started off with a very long title, which was Na Jane Kab Pyar Ho Gaya. However, it was later changed to a short and sweet sounding Hum Tum. Also, when the film started off there was apparently a cameo offered to Lara Dutta. However, according to IMDb, the actress was busy shooting for Khakee and hence had to opt out.
Jimmy Sheirgill plays a supporting role in Hum Tum. His character in the film goes to pick up Kirron Kher and Rani Mukerji at the airport. While in the car, they listen to some music. And the song that plays is 'Andekhi anjaani si' from Mujhse Dosti Karoge! This was also Kunal Kohli's film. The director has taken an almost unnoticeable reference from his previous movie. Mujhse Dosti Karoge! also starred Rani Mukerji in the lead role.
Saif Ali Khan plays a very quirky character in the film sporting colourful clothes and funky hairdos. One of Saif's dialogues in the film was, "This is my Tom Cruise look." Apparently, there is a story behind the same. According to IMDb, one of the assistants had said that one of the looks was like that of Tom Cruise. Director Kunal Kohli heard it and decided to make it a part of the script. While Saif didn't seem to be in support of it, Kohli managed to convince him.