As you head into working on your own projects, here are some of my thoughts on starter concepts for good interactivity.
** Reactive or Responsive? **
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Computers, and simple circuits, are best at REACTING to input from your user: an action triggers a single reaction. However, life and living beings tend to be RESPONSIVE: your actions become part of a dialog with those around you.
Creating reactive work can be beautiful, as we saw with Simone's piece, and it can have unexpected results for the viewer. However, sometimes your message is better conveyed by building a system that enters into a physical/emotional/visual (etc) dialog with your audience. While it's often very complex to make something feel life-like, or be truly and uniquely responsive to an individual, it's often worth thinking about, like Monika discovered.
As you think about something 'responsive' - observe your interaction with a child or a pet, or the subtle dialog with those around you on the subway - start thinking about the elements of input and output. It needn't be dry - the clues living creatures give each other are extremely complex and touch all our senses.

In my work, I find that little can be truly responsive - no response built of code and circuits can approach the complexity of human response, intention etc etc - but there are many subtle indications I can provide that suggest responsiveness.
- Immediate response
- Surprise
- Patterns
- Change over time
- Variance
** Immediate Response **
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Standing on that crowded subway, if you accidentally bump someone, they will respond immediately.
The immediacy of this response lets you know that the other being is very aware of you. In a piece, this immediate response begins a dialog with your viewer - much as the sudden darkening of Timmy's piece was a clear message to viewers.

** Variance **
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On the subway again, if you bumped that person, they might react in many different ways - anger, support, suspicion. This might be based on their personality, or something about you. Variance can be used to shape a more unique dialog with viewers, and it also suggests that your piece is making decisions.
Variance also comes into play if you ask people to engage with your piece over time. Few living creatures react the same way all the time, their responses are colored by repetition, your responses etc. Building into your system something that changes with repetition can be interesting - perhaps your piece is bored with this user, or shares something new after they have been there for a while, perhaps the visuals change - the options are endless and provide the viewer a reason to stay.
** Surprise **
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Surprise is an aspect of variance - and it's something living creatures do to each other all the time. My grandfather used to tell me that surprise is the spice of life and what keeps us on our toes. I think it's the same with good artwork too. I find surprise in all sorts of art - many of them completely non-interactive.
Imagine yourself on that subway, think of actions you might take that would surprise someone - a quick approach, talking to them, pulling something unexpected from your bag, smiling at them... how would they respond? How might they surprise you?

** Patterns **
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Surprise often comes from breaking an established pattern. Good surprises tend to create new and more interesting patterns. Bad surprises tend to seem to emerge with no reason.
Humans seek patterns in things around us - when interactive art creates patterns that are not discernable or too complex, people tend to get irritated or dismiss the work with a "well, I don't get it". Think about developing interesting patterns of response in your work. Think of how the patterns might vary with different kinds of users or user actions. Then think of the meaning those patterns suggest.
With all this, I suggest experimenting with the tools, sketching ideas, imagining impossibilities, iterating towards the impossibilities, and talking to others. Observation of people, like the subway, and of people with your prototypes is critical.

work by Casey Reas (his
site** Change over time **
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If the user stands in front of your piece for 10 seconds do they see the same thing as if they stood there for 1 minute?
If you seek to engage your viewers in an evolving dialog, consider how time is an element of your work and how the interaction might be shaped over time.

** User or Viewer **
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I use the terms 'user' and 'viewer' fairly interchangeably here. However, when I make work I do consider whether I want my audience to be users or viewers. Viewers watch a piece happen, but often, in my view, do not have much effect on it - they trigger playback. Users actively must engage with the work for it to be formed and to evolve.
Creating a piece for viewers tends to allow you more control over what they will see. Creating a piece for users opens your piece to much more interaction, and often the users will discover things you did not plan for. This can distract from your message or add to the joy of discovery in your work.
Either way, I find it helps to be aware of HOW I want people to engage with my work.

** Analog? Digital? Kinetic? Static? **
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Should my piece be....? Really it is up to you. But I suggest that simpler is often better: more robust, more elegant etc. But it's only better if it still conveys what you seek to convey.
I have been disappointed in the experience of work that I find technically amazing, and overwhelmed by work created with extremely simple materials and a beautiful sense of meaning.
Good luck! And post project here when you're done or want feedback!