the idea of reaction vs interaction, is one which is going to help me a lot while i try to develop 'interactive' projects now and in the future. it is a point brought up within the first two chapters of 'The Art of Interactive Design', by Chris Crawford. basically, according to mr. crawford, for something to be interactive, it must meet three points successfully: 1. it must accept an input, 2. process the information received through the input, and 3. formulate some kind of response, or output.
previously, i felt that if anything responded to you in any manner whatsoever, it could qualify as being 'interactive'. however, after thinking about the differences between reaction versus interaction, i realised that just because something delivers a response, does not mean that it did so by means of any kind of meaningful process. for example, if you sit on a chair, it will hold you up, exerting a force against your body (providing it can support your weight), regardless of any other circumstances regarding the event. however, if you sit on a person, they'll either be startled, or maybe laugh, or have no response at all. it depends on the context in which the event took place, and even though most of us would be weirded out by someone sitting on us, it's conceivable that there are circumstances in which it wouldn't be that strange (for example, in a crowded car, when it's necessary to play musical laps to fit 6 people in the backseat). this is a much more interactive event, by definition, because there is an array of responses available for a single kind of input.
anyways, it's definitely something worth thinking about.