In all fairness these days it might not be a notebook, it might be an electronic device (including Dictaphone etc), but either way keeping something to hand to record ideas is always helpful. The problem with ideas is that the subconscious tends to push them to the surface at the most inconvenient moments. For instance when you’re in the shower and couldn’t possibly have a writing implement to hand. Other times it’s best to have something.
The problem with not having anything to hand is that moment when you come up with that great idea, the line, the plot twist and you don’t write it down. Many a writer has been foiled by the absence of a pen and paper. That said, I always make sure I have a pen and notebook in my bag and yet, stupidly, don’t always write things down but I’m getting more disciplined at this.
Yep, I’m afraid it will take discipline to overcome the I’ll-remember-that urge because, trust me, in most cases you won’t. There can also be the worry that you might look a little odd stopping in strange places to write something down. I wouldn’t worry, I might get a gold medal in oddness when it comes to this. More than once I’ve stopped in the middle of examining shelves in the local bookshop to whip out my notebook and start writing without so much as sitting down. Assuming I could’ve found a seat to sit on in the first place.
Sometimes as a writer you’ve got ask yourself what’s worse: looking a bit weird or forgetting that great idea? Particularly when you consider that if you publish your work there will be a lot of people looking at it and judging it. If looking weird worries you then you might need to thicken your skin because you only have to open an Amazon page to see the sorts of things people will say about books. Look on the bright side, if they really piss you off you can put them in a story and some karmic retribution can occur.
Oh, and I would suggest, if you use a notepad or a memo block where the pages are liable to get torn out or fall out, that you have something you can keep them all together in. It can be worse to know you’ve written a great idea down and then you can’t find the piece of paper you wrote it on.