I am currently reading a very nice little improv book called Long-form Improvisation & the Art of Zen by Jason Chin, and though this book is full of really insightful stuff, there is one section that really caught my eye. Its all about creating a past for the characters in the scene. A theme that is repeated throughout the book is that the scene should be about the relationship of the two characters not what is actually happening. And how can you have a relationship without a past?
An impov scene should also be in the here and now, so how do you establish a past without dwelling on it? In Chin's book he suggests a few hints. You can establish a relationship with a simple name or term of endearment after a sentence. For example, "Would you like another drink?" would become "Would you like another drink, Tom?" or "Would you like another drink, you shit-bag of husband?" The key is to convey your point and history in a way that is obvious, yet not redundant or overbearing (one brick at a time).
The other thing Chin mentions is that conflict and argument are good. "A heated emotional exchange between two people is interesting if they're being led by their hearts." (Chin, LF&TAOZ, Pg. 14). However, a debate can ruin a scene. A debate is just a presentation of facts and is boring. I have seen allot of highly intelligent improvisers that thrive on witty comedic debates. The problem is that though you are showing off your smarts as a comedian you are generally not giving anything for your scene partner(s) to play off. There is no relationship in the facts.
I hope that this was helpful.