Q. What if a producer just wants to option my film idea? I.e. I have the one pager and he loves it and has initial interest out there and he wants to keep shopping it around with me?
You don't get paid for an idea. Sure, Joe Eszterhas optioned a napkin for $4M in the 90's after BASIC INSTINCT, but then the napkin film did not do as well as BASIC INSTINCT, and it is no longer the glorious 90's, age of the ridiculous spec sale. Anyway, you don't. You give the producer an informal, or formal, right to shop the idea, with the understanding, or written agreement, that you are attached as the first writer, at a reasonable (e.g. Guild minimum) writing fee.
Generally speaking, your right to write the film stops at either a draft, or two drafts and a polish, depending on your clout. After that, the producer can take you off the project.
A typical deal for a pro writer might be a Right of First Refusal to write two drafts and a polish for scale (or scale plus X% if s/he's an overscale writer), plus a hunk of money if the film goes into production (say 2.5%-4% minus what he's been paid already), plus a ROFR to write sequels/prequels/spinoffs/TV and/or passive payments in case s/he isn't the writer of the sequels/prequels. Plus monkey points
If you are a beginning writer, the producer may offer you only the production bonus, intending to give the idea to someone else to write. Don't accept that. The whole point of you shopping your ideas is to get to write them up. Vastly more films get developed than get made.
In the case of a one-pager, you're probably better off with, at a minimum, a written one-page deal letter clarifying that it is your idea and the producer is attached to it in the event he sets it up (finds development financing) within X time period. After that the producer is no longer attached, and any creative ideas the producer may have contributed are your property.
All of the above is technically redundant since you own the idea and a producer's doesn't own his notes once he gives them to you, but having it all on paper clarifies things for everyone, and if the producer has different ideas, you want to know them now, rather than later when he attempts to tie up your project without paying you.