Friends fans think they know everything about the iconic sitcom. And maybe that's true. But they also have to be true television fans to know that the show crossed over with another famous '90s sitcom, Mad About You. And that cross-over was done because of one actor... Lisa Kudrow.

Funnily enough one of Lisa Kudrow's biggest roles outside of Friends was a character she played on Friends... Phoebe's twin sister, Ursula. Although Ursula started out as having only a few lines on Mad About You, she became instantly recognizable and therefore the creators of Friends had to find a way of dealing with the fact that the same actor was appearing in two different TV shows, set in the same city, that aired around the same time. Here's the truth about Pheobe's evil twin sister...

Crossing Mad About You Over With Friends

Given that Lisa Kudrow was just breaking into the mainstream, there had to be some explanation for her appearance on two hit TV shows. It would be different if the two shows aired on different networks at different times... but that wasn't the case...

"When Friends got its timeslot on Thursday night at 8:30, after Mad About You, then they needed to start discussing - not immediately, but listen, 'I'm allowed to be on Mad About You, and I love Mad About You,'" Lisa Kudrow explained in her interview with Emmy TV Legends. "Everybody realized, 'Well, we need some explanation for the audience. Like why you're seeing the same girl with the same voice at 8:00 and at 8:30 on two different shows. We need an explanation.' I don't know if it was [Friends co-creators] Marta [Kauffman] and David [Crane] or [Mad About You creator] Danny Jacobson who came up with, 'They're twin sisters.'"

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Lisa went on to claim that she loved the idea that her character on Friends and her character on Mad About You were twin sisters. She thought that the two characters were perfect to connect that way as they were both two versions of a very unique, and strange character.

"I saw Ursula as this nice person. You know, really nice, just didn't pay attention to anything. And Phoebe was a nice person. She was fiery. There was more to her. I mean, because she had more than two lines," Lisa said. "She was a decent person but she was a little fiery. Not as dense and doesn't get things [as Ursula]."

To Lisa, Phoebe was someone who saw the world completely differently than most people. She wasn't dumb. She just had a different perspective, and a lot of that contributed to the secret of making Phoebe one of the most beloved characters in television history.

Making Ursula The 'Evil' Twin

While Lisa Kudrow never saw her Ursula character as a bad person, she definitely became that on Friends. If you look at Ursula on Mad About You, there are elements of her that are unappealing, but she is far from a bad person. However, if you watch Ursula on Friends, she is far from someone you want to hang out with.

"I think it was the Friends writers who decided that Ursula was this evil person," Lisa laughed. "Maybe they didn't like that they had to include her. I don't know. But I remember the first time. They were like, 'Alright, so here she is'. I don't remember what the story was. But I remember thinking, 'Oh, that's kinda mean. She's mean? I don't think she's mean.'"

Lisa claims that the Friends writers tried to pass Urusula off to her as nice but obvious. But Lisa didn't buy it. She knew that the character was being made to be a meaner version of Pheobe. This was probably done to further differentiate the characters as well as get the audience to love Phoebe even more than they already did. A character, by the way, who Lisa really enjoyed playing.

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But given the fact that Danny Jacobson and the writers of Mad About You had created the character of Ursula, chances are they weren't too thrilled about her portrayal on Friends.

"I don't know how Danny Jacobson or whoever was running Mad About You at the time felt about [Ursula's portrayal on Friends]," Lisa told TV Emmy Legends. "Because, initially, it was just to let me go out to do Mad About You and do [Friends]."

Around the same time, NBC was really into the cross-over potential of their shows. They believed it was a way to further engage audiences and get them to tune in to more of their shows.

"Because all of these shows took place in New York. Mad About You, Friends, Seinfeld," Lisa said.

Another way they did this was to include a blackout that the city was experiencing. This was another way that Friends and Mad About You were connected to each other, both had episodes dealing with a New Your City-wide blackout. But, of course, they also brought on Helen Hunt's Mad About You character onto Friends for a brief cameo where she mixes up Phoebe for Ursula.

When Mad About You ended in 1999, Friends had more of a license to make Ursula into as much of the 'evil twin' trope as they wanted.

Regardless of whether or not Usurual was supposed to be as cruel as she ended up being in Friends, it appears as though Lisa Kudrow had a blast playing the character.

Next: Here's Why Lisa Kudrow Is The Smartest 'Friends' Cast Member