Nathan Fillion wore his heart on his sleeve in an emotional message to a loved one which fans couldn't get over.
The star took to social media and wrote a gushing tribute to a very good friend who he says it is "an honor to know".
Alongside a photo of the actor, Michael Rooker, Nathan penned a lengthy message which read: "This is @michael_rooker. I can't tell you what a pleasure it is to work with someone so talented, kind, generous, and energetic as this man.
WATCH: Nathan Fillion looked so different at the start of his career in Castle
"He truly loves his craft, and he makes the hardest work look easy. Not only have I had the pleasure of working with him on numerous occasions, but I also have the honor of calling him my friend."
Nathan concluded: "I hope your birthday brings you joy, Rooker. I appreciate your friendship more than you know."
Fans adored his kind words about the Guardians of the Galaxy actor and commented: "@michael_rooker he is a natural talent. Happy birthday great and talented man," and, "I met him at Comic Con, one of the nicest people I've ever met. Happy Birthday, @michael_rooker may this you be full of good times, love & adventure".
Nathan had lovely things to say about his friend Michael Rooker
Many more wished him a happy 67th birthday and there was plenty of surprise that they were such great buddies.
Others remarked that they didn't recognize Michael in the photo, and some said he hadn't been on their radar - but he is now.
Nathan is currently reveling in the news that The Rookie has been picked up for season five. He shared his joy on Instagram and captioned a poster from the show with a sweet message.
Nathan is delighted that The Rookie has been picked up for season five
"I couldn’t be more proud of what we have accomplished with this show," he wrote. "Congratulations to our cast, crew, writers, producers, casting directors, directors, and everyone who has lent their talents to #therookieabc over the years.
"This job brings me real joy, I look forward to many more episodes with you all. I'd also like to thank the fans who keep coming back. You keep watching 'em, we'll keep making 'em. #therookieabc #season5 #holycrap And thanks to ABC for the early pickup! It's such a good feeling."
Read more HELLO! US stories here
Like this story? Sign up to our HELLO! Mail newsletter to get other stories like this delivered straight to your inbox.