Horgan remembered Fisher, who died at 60 on Tuesday, by sharing a recent photo of the two smiling widely on the Catastrophe set.
“Carrie was my friend. It took me three series but I got her in the end. She was the most generous, fun, gifted, smart, kind, funny funny funny person I've ever met,” Horgan wrote on Instagram. “She certainly wasn't ready to go.”
“I'm so glad we became pals,” she continued. “I'm so devastated at her loss. I want to write about her more but I can't process yet."
Fisher had shared the same photo days earlier on Dec. 19, writing on Twitter in her own unique voice: “Sharon Horgan & I pretending we like each other while filming Catastrophe in London.....Succeeding!”
That same day, Fisher posted another candid photo with Horgan and their dogs, including her beloved French bulldog, Gary, as well as a sweet moment with David Cross.
One day later on Dec. 20, Fisher snapped a photo with a fan on the streets of London outside the Salvador Dali Museum looking healthy and happy.
Alongside photos of Fisher sightseeing that same day, Fisher -- who was reportedly promoting her book, The Princess Diarist -- joked about her cameo in Rogue One: “Gary heard I do some of my best work in it.”
Earlier this year, ET spoke with Horgan about the chances of moving Catastrophe from its U.K. setting to the U.S., as well as Fisher’s pivotal role on the show.
"We’re very in love with Carrie Fisher, so we always try and think of how we can get her in it more. So, obviously, heading back to Rob's hometown would give us more time with her,” Horgan told ET back in August. “And so it is a little bit of, it's something we can confer for a lot of reasons. But, you know, hopefully at some point it'll happen."
“It is with a very deep sadness that Billie Lourd confirms that her beloved mother Carrie Fisher passed away at 8:55 this morning,” family spokesperson Simon Halls said in a statement to ET on behalf of Fisher’s daughter, Billie Lourd, on Tuesday. “She was loved by the world and she will be missed profoundly. Our entire family thanks you for your thoughts and prayers.”
For more on Fisher’s death, watch the video below.