Every question has a meta question behind it, so every answer should have a meta answer behind it as well--ie, take a moment to think why they're asking you this question, and be sure your answer satisfies what they're looking for.

The "What's your biggest weakness" question isn't asked because they want to know that directly, they want to know about how self-aware you are about potential shortcomings and if you are taking steps to improve yourself, etc.

Structure answers to keep from rambling. I like to use STAR: Situation, Task, Action, Result. Clearly state what the situation was for context about everything else you'll say. What part did you play specifically in the plan? What individual actions did you do as your part (and be sure to say "I" as much as possible, saying "we" can make it sound like you were just along for the ride and didn't do anything yourself). Lastly, talk about the outcome and how your actions contributed to it being positive, and anything you've learned to use in the future.