A Different Kind of Contract

Do you mind? This is a private conversation.
Whenever someone sits down to read or watch a work of fiction, they are forming a pact with the author. Two, in fact. The first is what is commonly known as 'suspension of disbelief'. That roughly translates as 'I won't question anything you tell me, as long as it makes sense in the context of the story. Time-travel, unicorns, a different end to a war, alien robots that look like chairs? No problem, just as long as you can convince me with your writing.
The second contract is similar to the first and equally unspoken. Ask for my time, dear author, and you better damn well deliver. Sign here in blood, please. I want to be so involved in your fictional world that I miss it, and think about it, days after The End. I want to feel the story, be swept along by the narrative and plot, and see the world through different eyes as a consequence. (Only for a day or so, mind. Any longer than that and we'd all start to worry.) I want believable characters, authentic dialogue, and a plot that surprises and satisfies... Oh, and did I mention originality? I don't care that there are only seven plots - give me something new (and even if it's a sequel, it needs to stand on its own literary feet).
Recently, I watched a film on TV. Not a bad film actually, taken in isolation. However, it was such a breathtakingly blatant rip-off of a previous (and better) film that I'm amazed they weren't sued for plagiarism or prosecuted for making counterfeit goods. I won't name names, but I'll just say it was a thriller, and let's leave it at that.
So today's questions are:
1. How do you ensure that you're not blindly following someone else's trail creative (unconsciously, of course)?
2. When does homage become plagiarism?
Discuss...