In one of the writer's groups to which we belong the conversation dealt with how to make fictional scenes and characters more interesting.  A technique for heightening conflict is to build in subtext. This is what goes on behind the words characters are speaking.

One way to insert subtext is to put the characters into a situation that shows something opposite or different from what they are saying. For example, ostensibly the characters are talking about what a great relationship they have and how much they are in love.  But the scene is set in front of a construction site.  Every time they try to speak,
jackhammers start crashing.  Construction workers shout.  Chaos reigns.  The subtext is that this relationship, despite words of endearment, is either already or about to experience turbulence.

How do you write subtext on the run?

Remember, life is your page.