Carrie Fisher's Friend and Costar Sharon Horgan Reveals the Actress' Final Hours Before Fateful Flight — and Last Gift for Mom Debbie Reynolds

Just hours before her tragic flight from London to Los Angeles, Carrie Fisher invited Catastrophe costar and co-creator Sharon Horgan up to her hotel room so she could give her one last parting gift before heading home for the holidays after wrapping season 3.

“The last time I saw her was in her hotel room before she took that flight back to L.A.,” Horgan, 46, tells PEOPLE of their time together. “She brought me up to her room because she’d been in a little antiques market that day and she’d gotten me a beautiful little cocktail stick holder.”

Fisher, whom Horgan remembers as a “prolific present giver,” also showed her a ring she had picked up for her mother, Debbie Reynolds.

“She was never buying for herself — she’d always be picking up little pieces for other people,” Horgan says.

Shortly after saying their goodbyes, Fisher boarded her final flight home. She suffered a heart attack mid-air, and died several days later in the hospital. (Reynolds died one day after her daughter, on Dec. 28.)

RELATED VIDEO: Catastrophe Season 3 Sneak Peek with Carrie Fisher

Now, with Catastrophe‘s third season debuting on Amazon Friday, Horgan is opening up about her memories of working with the legendary actress — and how she’s coping with the loss.

“The really tough time was obviously first finding out about her heart attack, and then finding out that she didn’t pull through,” she says. “That was horrific.”

“And then I think the next difficult thing was being in the edit [room] and watching her on screen yet no longer being with us. That was really tough because she was so alive on screen, and we had wrapped with her just days earlier. It felt surreal — it didn’t sort of make sense,” she adds.

After taking a larger role in the third season, Horgan says she formed a fast friendship with Fisher.

“We spent a lot of time with her this season filming and not filming, and got to spend a lot of time getting to know her — what an incredible person she was and what a sort of lovely, kind, generous lady,” she says. “I knew she was funny and interesting and all of those things, but to know her on a personal level a bit more — I realized how much more to her there was.”

Catastrophe tells the story of a couple who decide to get married after unexpectedly becoming pregnant, and Fisher joined the cast after Horgan and co-creator Rob Delaney were looking to add a character who would play Delaney’s mom.

After seeing Fisher present at the [U.K.] Attitude Awards, Horgan thought, “She made this incredible speech that just brought the house down, and she was just so vibrant and killer onstage.” She says Delaney also knew right away that Fisher was the right actress for the role.

Still, after sending Fisher the script, they held out little hope that she would accept the part. When she actually signed on, Horgan says, “We were so shocked to have her in the show that it took us awhile to start seeing her as a human and not as an icon, someone so far removed from our lives and our world. We were just sort of dumbfounded.”

But it didn’t take long for real relationships to form. When Delaney couldn’t attend a screening of the second season at Tribeca, Horgan asked if Fisher could fill in, never expecting she would say yes on such short notice.

“She was getting an honorary degree from Harvard, and she drove all the way down with [her dog] Gary on very little sleep to walk in and do this little screening with me,” Horgan remembers. “But that was the kind of lady she was. And that’s when I started to get to know her a bit better, and it would have been lovely to get to know her even more. It was at that point in a friendship when you think, ‘Oh my gosh, you’re in my life now.’ And this is great, it’s all the better for us.”

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Horgan was looking forward to Fisher having an expanded role in season 4, but after her death, it’s “now about honoring her character in the next season. Trying to find a way to deal with it without being morbid and sad. It’s going to be hard to do it justice but we’re going to try to do it in a way that feels right.”

Remembering Fisher, Horgan says one anecdote always sticks out in her mind. “I remember she was on set one day and we were filming with the actress Frances Tomelty. Frances said when she’s choosing a scent or perfume on set, she has to be careful about it because she doesn’t want to wear anything too overpowering or too cloying.”

Fisher’s response? “I f—— love being cloying.”