So... players are the real enemy of every plan, doesn't matter how well you plan it.
Definitely. One of the most memorable occasions was when we did Frozen Reaches (somewhat modified for a kill-team rather than a Rogue Trader Dynasty, but it works well). One of the players, finding out about this mysterious relic, decided to steal it. But asked if he could do so without telling the rest of the team. We had a seperate evening where we ran through the heist (he succeeded), and then he continued to not mention the fact he'd taken it to the rest of the team. Despite a borderline civil war kicking off in the capital as accusations were flung back and forth between the various rogue traders.
Best suggestion for getting them involved - get them involved in planning the mission more. Our gaming group tends to take 3-4 weeks per mission:
Week 1-2: Experience from last game, general 'state of the war' briefing, mission objectives outlined for the next mission, Map (sketched) and as much information* as they get before arrival. They then outline the plan, from insertion to fiendish tricks, and are encouraged to come up with as outre suggestions as they can. Then requisition, to gather up the kit needed for these shennanigans.
Week 3-4: The actual mission.
The more they're involved in planning the mission (as picked veteran special forces should be), the more they will (a) get invested in the mission and (b) you get at the same time helped in coming up with the story - you essentially need to produce the setting, the objective and the twists, they generate the rest.
just be prepared for the players to go off your flow chart…..(one of) a gm's most valuable skills is being able to wing it…..
I have produced a flow-chart for a lot of my missions. I find it amusing how many of the boxes have a line running down to the right hand corner to a box marked "gruesome death".
* which may or may not, of course, bear the slightest resemblance to accuracy, depending on how recent it is, how much effort they put into confirming it, etc, etc.
The kill-team previously decided not to spend the 15 requisition required for orbital recon before launching a major assault into infested territory on Castobel during the last mission. This resulted in their tactical briefing not including ~ 1,000 magnitude worth of tyranid brood creatures, supported by warriors and heavier organisms in the combat area, and cost them about half the guard regiment they brought with them. They have since learned that knowing is in fact significantly more than half the battle, and that any option for reliable recon should be siezed upon like a starving man upon a steak platter.