Secrets and Lies

How do you make an Improv drama?
Where would you even start?
Dramas have thick plots and specific things that have to happen, a storyline and everything..
I guess you would just assign characters and let the actors get to be the characters.. But most of the improv games Ive ever seen or played or read about are very comedy-oriented.
I guess it would just be "ok, youre a very stressed mother trying to make conversation with your daughter who wants nothing to do with you. Go play a scene." But that would only work for some of the scenes.
I think I'd have to watch a lot more imrov dramas to see a similarity in them.. Or at least more dramas in general. Comedy is more my preferred theme.

I really wonder how the director/writer of Secrets and Lies went about making the movie. It seems like a very interesting process.