Some good advice already given. The few that occurred to me reading the OP:
1) Leave your "expertise" behind. Doesn't sound like there is anything there to be the reason they are hiring you. Not that the experience isn't important, but you even recognized you are not an expert. Instead, go in with a learning attitude.
2) If you want to advance in your career as a climber, the most important thing to learn is about trees. (at least in my opinion). Remember, the point is NOT to climb a tree. The point is to care for (or remove) the tree. Climbing is only the tool. Too many hacks out there that think because they have the ability to get into a tree (whether via climbing or, more often, with a bucket) that they should be trusted to care for the tree. So...even in the first 2 days, start learning about trees.
3) You don't need to show off, but decide that nobody is going to outwork you. Not only your first 2 days, but every day. Doesn't mean you are always running 100MPH...think more like the tortoise and the hare. Just don't quit moving forward. You can live 2 days without your phone. Make it easy and leave it in the truck.
4) Don't brag about your free climbing. Safety is a very, VERY high priority for most reputable companies. Not only being/acting safe, but more of a safety mindset. If I knew you were illegally) free climbing train bridges and water towers I'd have strong doubts about your safety mindset... It does sound fun, I'll admit that, but just sayn'...if you want them to have a picture of you, think about what you want them to include in that picture.
5) Respect others' property/tools. Whether the company's or the client's. Care for it like you just bought it and want to return it to the store when you are done, pretending like it was never used.
6) Have fun!!!
Most of us don't even use the same number of syllables, let alone same vowel sounds.
