The pair have often been rumoured to be at loggerheads, and some outlandish reports even tried to allege an affair between them.
But speaking on Jimmy Kimmel Live last night, Dornan – who plays Christian Grey in the erotic movies – explained that they really have more of a sibling-type bond.
“This is gonna sound bad but it’s almost like brother-sister,” he said.
“We just… cos I am married, she’s had quite a lot of relationships in the time we’ve known each-other… so that’s happening in our own lives.
“And you have this mutual love and respect for each-other. We know each-other intimately.”
Fifty Shades Freed is the third movie in the series, and seemingly the final one – and over the years, Dornan and Johnson have had to shoot a very large number of sex scenes.
Fortunately, however, he said the experience was made slightly less awkward by the absence of huge numbers of crew members on set.
“There’s some [crew members] that carried over for all three [movies],” he recalled. “They try to keep it a closed set for the sex stuff.
“Only whoever really needs to be in there. Like me!”
Dakota Johnson has also been on the chatshow circuit: she told Jimmy Fallon earlier this week that there will be no more movies in the franchise, despite claims to the contrary.
Fifty Shades Freed is out on February 9.