In a romance story, it's often assumed by the reader that the two main protagonists are going to be involved in...well...a romance. Duh! In some of the stories I've come across lately, I've noticed a lack of romantic interplay. Therefore, I'm in the mood to opine!
In the case of a romantic suspense story we often have characters on the run or heavily involved in a dangerous mystery that could lead to the destruction of all humanity! This doesn't mean they won't take half a second to notice each other's attributes. Face it, they're human (or if not they still think of the same thing we do!) and are going to look at someone they're interested in, whether or not they're running for their lives.
When a hero is panting from an exhaustive run toward or away from a bomb or tentacled bad guy, a glance toward the heroine's deliciously, disheveled 'do and down her curvaceous form is perfectly acceptable. In fact, I completely encourage such behavior! (In more ways than one, but you probably knew that already!)
The connection between characters starts with the visual and moves through all the senses. Heightened senses due to adrenaline often absorb more information than less. This leads to all type of actions and reactions. Don't waste the moment just because you are too involved in the choreography of the action scene...remember the play in interplay!