Kevin Costner proved to be a stand-up guy during an intimate scene in his new movie … ’cause his costar says he brought a sensitive nature to filming the down and dirty.

Abbey Lee — who stars in Costner’s new movie “Horizon” — dished on going all the way with Kevin onscreen in an interview with People … and, she says the veteran actor brought a great deal of sensitivity to the interaction.

Lee says Costner was aware of how daunting sex scenes can be … adding he wanted to make sure she felt comfortable while they filmed — and, sat down for an extra-long convo about all the beats of the scene before they started shooting.

Costner made it clear to her there’d be no nudity in the scene, she explains … adding the full conversation made her feel a little bit extra comfortable when they shot it.

That said, Abbey admits there’s always a small feeling of discomfort in these intimate scenes … especially ’cause the crew has to look on at this fake, private moment. Just a super strange feeling in her view.

The new flick opened this weekend BTW … and, the reaction’s been lackluster, to say the least. It’s only made $11 million at the box office and it’s got a 40% on Rotten Tomatoes — so a shaky opening on both counts.