This twilight period, before you commit to a full treatment and are just spitballing, is possibly one of the most fun parts of the process. Here, your creativity is only limited by your imagination - you don't have to make considerations with regards to usual script standards like act length or commercial breaks - you can just focus on caputring those 'wisps' as Ed Solomon terms, those anomalous little ideas that slowly bring your idea to life.
It's also a good time to ask yourselves questions about your ideas: interrogate each and consider what the challenges are, and if they actually stand up. Using questions can also help ignite more creativity and find solutions or even better versions of these ideas. With an adventure series, the obvious places concern how and where the heroes will go/get to, what the goal is and how do you play and build up that lore? Issues around travel logistics or clue placement or the use of setpieces, and what kind, arises.
While I've done most of this with the ol' pen and paper, I have also opened up a Word doc with the intent of having something where I write more longform ideas, without having to commit to the more judicious treatment yet. This is a place where I can experiment with world building, write up character bios and backstory that may/may not appear in the final story. I can also see how certain ideas, when laid out in more detail than above, play and if they could fit into further expansions and breakdowns of the pilot. Call it my 'dumping bin', my 'waste basket', my idea store' - point is, it's handy.
Some of these questions have included:
- I have this historical event connected to the Romanovs, and I want to go in this direction. How can I work with and around the facts of their lives, especially their last days in the run-up to their execution? How can I have them do X or Y while keeping to that canon?
- I want their to be a big old treasure hunt, with clues in unusual pages. How and where can I put them and tie them into the Romanovs? What do i know and what do I need to brush up on?
- So I want my protagonist to have had past glories, which clash with his present? Where do you go with someone who is adventurous in youth, without just repeating 'being a total neb' like in Hook or Christopher Robin?
Of the two, the shift is more noticeable in B&M, where it takes more of a science-fantasy dimension as the series progresses, as opposed to the more standard sci-fi and spy thriller angle of the comics (especially after Jacobs died and other writers took over). They deal in Vikings spectres, alternate realities, alchemists and even bringing in a new character, Mortimer's female cousin, who's a type of paranormal investigator. It's not bad, far from it in fact, but it does come off as a little dissonant compared to where the books went in the new century.
Still, being mostly half-hour two parters, they get in and out quickly, hit the key beats and deliver a satisfying story, with a clear ending cliffhangar (though of the two, the Tintin series was better at them than B&M).