A: First of all, at a baseline, there's different type of physical training. Before my first time on Survivor, I spent a lot of hours in the gym - bulking up with muscle strength. I quickly learned that that kind of power is irrelevant on Survivor, and plus you're adding a lot of mass that will just atrophy when you're deprived of calories. You need short burst endurance and speed, as well as balance (which I am terrible at, and no training will help me). For my second time, I did a lot of Crossfit-style, full body workouts that lasted 20-30 minutes of sustained effort. Though I certainly looked like a challenge clown when it came to later contests of balance and precision, I found I was in great shape for all the more physical competitions I participated in. I definitely, definitely, definitely would practice more balance and coordination if I were ever to do Survivor again. I think returning players also have a better sense of which type of puzzles to practice.
I also ate differently. First time out, I really tried to mass up a lot - not just by working out but by drinking lots of high-caloric protein shakes and eating a ton of carbs. Second time, I worried about this less. I ate fat-rich foods like avocados and peanut butter, but I didn't want a lot of bulk which again would just atrophy. I found the hunger MUCH more manageable my second time, going in leaner, though part of that might be a difference in expectation - I already knew how hunger would feel. (My first season, too, we were uniquely starving I think because my tribe missed out on most of the food).
Social/strategic preparation is a trickier question. My first time out, I watched a lot of old seasons looking for general play styles that different types of players adopted. My second time, I was looking for specific intel on the different players I would be interacting with.
In a returnees season, you have a decent idea of who may be out there with you. You have the option of making pregame alliances or not. I have typically been of the perspective that pregaming can hurt you more than it helps you by locking you into agreements before the game begins, so I avoided it before my second time in Cambodia. However, I'm now a little more uncertain about that. I think pregaming will affect the game, positively and negatively for different contestants, and I don't think you can tell beforehand how it will impact you. It's dangerous for sure.
A: Adaptability is definitely a top attribute. Things change quickly, and you have to be able to accept that your plans may change too. That could be from a huge shift in the game - say, a swap or the merge - or it could just be that you learn some new intel. You have to be able to change your plans and not get locked into one way of thinking about things.
You also have to be observant - and trust your observations. Having great reads on people may be the MOST valuable skill. Look at someone like Tony Vlachos, who won Season 28: Cagayan. He had the superpowered ability to read people, so even though he would make erratic game moves, he could accurately assess how they were impacting the people around him. You also have to be able to really trust those reads - I had a few examples in my second season, where I noticed something awry with Kelley Wentworth before she played her idol, and Spencer Bledsoe before he backstabbed me, but I wasn't able to articulate it, and just dismissed it as background paranoia.
Self-control is also important. Being able to look calm when you're really paranoid. This is something I was good at my first time but not good at my second. People are attracted to confident and relaxed allies - paranoia just breeds paranoia. Spencer, for example, was very good at seeming confident, and that gave him allies across the spectrum.
A: 99% of Survivor is below the surface. On-air you see the tribal councils, the challenges, maybe a few key goofy moments, and a lot of strategy talk. But ultimately you're only seeing about ~10-15 minutes of camp life for every 72 hours that's filmed. There's a lot of boredom, lying around, chatting about families, making dumb jokes. If you go back and watch the Worlds Apart finale, when Jeff showed the unedited footage of Dan making an offensive joke, you get a sense for how most of the day is spent. Long, slow conversations with big pauses between someone saying something and anybody responding.
There's also just a TON more strategy talk than you see. I think people wonder - and I used to wonder - "Hey, why is player x not talking to play y." I will almost guarantee for you that everybody talks to everybody, at least a little. You're living with these people 24/7. They just show the conversations that are most relevant to the story at play.
There's also just much more daily maintenance that goes into building an alliance and pulling off a plan than you see. From the perspective of the show, it's one or two conversations, a handshake, and then the trigger is pulled. It's actually twenty or thirty conversations, tons of small reinforcements, little moments of support and reassurance.
And of course that's also how conflicts brew as well. We just see the on-air explosion, not the buildup of hours and hours.
And then there's the night time. The longest nights I've ever had have been on Survivor, where you literally watch the moon cross the sky.