I love a great Del Close quote. If you’re not familiar with him, he’s one of the forefathers of long-form improvisation, and he helped develop the primary theater where I studied, the iO Theater (known in his time by its former name, the Improv Olympic), into the improv powerhouse that it is today. In the case of the above quote about not being apathetic in your scene, it means to take on a point of view or emotion or really anything to give the character you are creating on the fly depth, which in turn creates interest in the audience. It’s such a simple concept and sounds like it’s easy to do, but it’s really not. It takes a lot of practice for this concept to become second nature in an improviser.
On a related topic, one thing I hear time and time again in improvisation is to do something, do anything, just don’t do nothing! Make a choice to give your character something to do or something to believe in. Even if the only thing you can think of to do is to flip a pretend coin in the air repeatedly, which may not seem to have anything to do with what is going on in your scene, you’ll probably find a way to tie it in by the end. And frankly, even if you didn’t, it gives your character a little quirk for the audience to see, and it makes him or her much more believable than a character standing on stage with their arms by their sides!