Does doing nothing make you complicit?

A question that has come up in two comments I've been pondering my reply to ¹ and on which I think I may actually have a really basic ethical difference about:

Lets assume a certain act is "bad" under your ethical code. It's hurtful, unethical, immoral, etc. If you did it yourself you would be being immoral/unethical.

If there is something bad going on, and you're aware of it, and you could (try to) stop it, and you don't, are you complicit in that bad act? Are you being somewhat immoral/unethical?

Because I say yes. Inaction is itself an action. It's not the same as doing the "bad" act yourself, but it's not completely different either.
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So, do people agree? Or do you not see inaction as just another form of action, subject the same moral/ethical rules (whatever they are, depending on your own POV) as, uh, active action? (You can tell I never studied philosophy, there's probably proper jargon for this stuff) Is there some hole in my argument or description?
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