Job: Acting, Singing, DancingI spent ten years or so (from age 12 to 21) studying the performing arts in various forms. I played instruments, worked as a dance coach, performed in plays, sang in choirs, and spent my first two years of college studying classical voice.
Most of the lessons from each of these disciplines apply to the rest as well, so I'll just write these up as though they were all one job.
1 - Lazy practice makes for lazy performanceA classic problem among performers who are just starting out is that they feel embarrassed giving a full performance in rehearsals, preferring to "save up for the show." It feels strange during rehearsals to break into tears or to passionately kiss someone, but the people who hold back for weeks before a show are never really able to commit fully on the big day.
In any activity, the habits we build from day to day condition our mind and body to perform those actions by reflex later on. In game design, I think it's very important to hold ourselves to the standard of whichever role it is that we want.
If you want to become a professional designer, lead designer, etc., don't wait until you've achieved that role to start thinking and behaving like one.
2 - A shared vocabulary is importantWhen watching an expert group of dancers or musicians rehearse, it's astonishing how quickly they can communicate. Various forms of music and dance all have their own highly specific language to aid in communication. A musician who has learned their discipline, but not its language, will never be able to collaborate successfully, and this is also true for game designers.
3 - Small goals are more satisfyingBecoming a virtuoso dancer, actor, or musician is a such a gradual undertaking that it's almost incomprehensible. After a day of rehearsal, it's impossible to really know if you've gotten any closer to your eventual goal - it can take decades.
Performers solve this problem by focusing on much smaller goals: learning choreography to a specific dance combination, practicing scales, learning new songs, etc. Game designers need to apply this same strategy when it comes to making sure that players feel satisfied. Doling out small tasks that lead toward a larger goal is something that RPG designers do very well, although it can be difficult to not overcompensate and give the player such small tasks that they never feel they've achieved anything.
4 - Reputation counts more than skillI once saw one of the dancers in my troupe drop his partner on the head. He was generally a very good dancer, but he had quite a hard time finding partners for the next few months. All the girls saw him as someone who might injure them.
Game development is similar, in that actual skill level is almost completely ignored in favor of how easy people are to work with. Or rather, skill level is important to your reputation, but can be cancelled out by having a reputation for being hard to work with.
5 - Smart leaders delegate to trusted expertsDirecting a play is a ridiculously complicated job. No one in their right mind would attempt to handle staging, set construction, lighting, music, choreography, and publicity on their own. The best directors find trusted experts, and shop out responsibility and power to them. Those leads then delegate further: The musical director is appointed by the director, and in turn appoints an orchestra leader, a vocal coach, and a choreographer. The choreographer then appoints a dance captain.
When successful, both plays and videogames are the product of multidisciplinary groups, unifying their efforts toward a common goal. Wagner referred to this as Gesamtkunstwerk, or total artwork. Strong creative leadership is essential to such a synthesis, but micromanagement is destructive.
6 - Know when to blend inThere are times to show off, and times to be part of the group. Singing ten-part harmony in a chorale is very different from singing a duet, which is very different from singing a solo aria. Likewise, dancing as a Rockette is very different from dancing as a prima ballerina.
Great performers always take advantage of their moment to shine, but also know how to collaborate unselfishly. Game designers are particularly bad at this. Ask yourself how important the feature you're working on is. How central is it to the game as a whole? Your painstakingly-simulated weather system may not be warranted in a game about riding dinosaurs off of sweet jumps.