Joosten passed away on June 1 at the age of 72. The actress played Karen McCluskey in all eight seasons of Desperate Housewives.
"Kathryn Joosten was an amazing actress, but an even better human being," Longoria told journalists at the Monte Carlo Television Festival. "She was an activist, she cared a lot about a lot of issues.
"The last day of filming when we saw her, she was great. She was in high spirits, she was laughing. She didn't seem near the end, so it was quite a surprise for all of us once we wrapped the show that she passed. She's deep in our hearts and we're going to miss her terribly."
Longoria also said that she developed a friendship with her Desperate Housewives co-stars despite being a relative newcomer to acting when she first started on the show.
"The girls are great. When we started the show, Teri, Marcia, Felicity, they all had shows before and so I was kind of the new one," Longoria said. "They really took me under their wing and embraced me being the baby and taught me a lot about the business, about acting, about friendships.
"We've all been through so much - marriage, babies, divorce, cancer - we've just evolved together. And to go through something so crazy like this show when it hit, we only had each other really to relate to.
"We had photoshoots at 4 in the morning, it was crazy, and we could only really complain to each other because nobody really wanted to hear us complain! But they're great. They're a good group of girls."
Watch a clip from Desperate Housewives below: