There’s just one rule that I know of for writing a romance into an action-focused novel and, unfortunately, a lot of authors don’t seem to know it.

Don’t force your romance sub-plot into the midst of the action where it doesn’t belong. 

Don’t have your hero and heroine getting all sexy or mushy while bullets are flying and they are fighting for their lives because that’s not how real life works. People first pay attention to staying alive and fall in love only when the danger, though it may still be real, is not so critical as to push everything else into the back of their minds and/or emotions.

Save romance for when and where it would fit in a similar situation in real life.