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Laurie Forman was originally intended to be a main character, as the 7th teenager. Lisa Robin Kelly (who plays Laurie) abruptly vanished after the first half of season 3, and was cut from the opening credits for the rest of the season. Some sources claim she left on her own initiative, others that she was fired, despite being quite a popular character on the show, so there are two very different stories out there about her departure. She returned back to the show toward the end of the fifth season, and appeared in a few episodes, only to disappear again at the beginning of the sixth season, and then was replaced by Christina Moore. Her character was eventually written out after season 6. Just over ten years after Kelly's last episode as Laurie, Kelly died from multiple drug intoxication (an overdose). It is believed that Kelly's own issues with substance abuse and her psychological issues (and a miscarriage) prevented her from committing to the show full time.
Mila Kunis was only fourteen when she auditioned for the show. The casting directors required that any interested actors be at least 18 when auditioning. When producers asked her age, she misled them by saying, "Well, I'll be eighteen on my birthday." and neglected to say what year.
Mila Kunis (Jackie) had her first kiss in real life at the age of 15 as an actress on this show with Ashton Kutcher (Michael Kelso). She and Kutcher got married years later and have two children.
The filming of the final episode, "That '70s Finale", was frequently delayed because Laura Prepon (Donna) couldn't stop crying. She had become extremely emotional as the realization that she would no longer be filming the show with the cast and crew she had started to consider as family, hit her really hard.
Debra Jo Rupp (Kitty Forman) wears the same red blouse and floral skirt in her final scene of the final season as she did in her first scene in the first season.
Topher Grace was cast after the show's creators, Bonnie Turner and Terry Turner, saw him in a high-school play. They had attended the play to watch their daughter and were impressed by Grace. Grace had never acted professionally before.