For those interested, this is from the Fireproof website, under their "Frequently Asked Questions" portion:
What’s the deal with the kiss between Caleb and Catherine in FIREPROOF?
In a romantic scene in FIREPROOF, lead actor Kirk Cameron is actually kissing his wife Chelsea rather than Erin Bethea, who plays Catherine. Chelsea was flown in from Los Angeles just for that shot. The scene was filmed in silhouette and works incredibly well, with Chelsea wearing the same outfit and a wig to mirror Erin. Sherwood Pictures has upheld this standard in all their movies: actors and actresses should guard their own marriages while on screen the same way they would do in real life. That’s why you don’t see Coach Taylor (Facing the Giants) or Jay Austin (Flywheel) kissing the women playing the on-screen wives.
Kirk also has long held to a principle that he would kiss no woman other than Chelsea. He was able to honor his marriage while making a movie that inspires others to do the same.
Imagine that, friends! Movie-makers and actors with morals! :o)