What are the required skills of a session Steel Player?

[Johnny Cox]

Luck

[Pete Burak]

Here is a Vid featuring Buddy E, on the topic of Improvising in the the Studio:
https://www.youtube.com/watch?v=flXaj4GWam8

[Ian Rae]

Listen to string quartets and make sure you understand what the viola is doing

which is an excellent suggestion for training your ears to hear inner parts. The most valuable part of my musical education was when my voice broke and I sang alto and then tenor in the church choir. I've been confident about singing at sight ever since (and at playing brass instruments which equally require you to hear the note before you sound it).

you can have your take and edit, too!

which is terrible and I'm only repeating it to prove I have the guts.

I would throw into the mix the ability to read music. 99% of the time it's no use, but there are people who set their musical thoughts down in notation and if you can read what they've written it saves a lot of time. Buddy Emmons famously never learnt to read music, and I'm not suggesting that this hampered him in the long term; but it seems there were a few times when his career might have taken a different course.