Ed McMahon had a great quote on a recent TV documentary about Johnny Carson. He said, about being a sidekick, "You had to be good, but not too good". I've been there, done that in advertising.
I've paid my dues as the sidekick to a variety of creative directors, each with their own priorities and their own schtick. It wasn't easy.
As Associate Creative Director, I worked for one CD who simply left it up to me to get all the department's work done. He knew I could be depended on to get everything done, even if I had to do a ton of it myself. He said I was "the perfect combination of the Protestant work ethic and Jewish guilt". He spent most of his time at lunch.
Another creative director loved my work, discussed his problems with me --- until his envy finally emerged. After a meeting he threw a dart into one of my layouts he had presented in the meeting. Being the strong, silent type, I never did tell him why I quit.
Still another creative director I worked for simply over-creative-directed. Everything had to be exactly his way. He came to every meeting equipped with two red pencils and two red pens. He re-wrote people's copy on the spot. When I left he offered me a huge raise to stay. But it was blood money.
Throughout these tenures, I learned to be "good but not too good". It had its rewards. Good pay, a good title, a good amount of respect. I agreed to be second banana in return for less responsibility. I could always duck and let my boss take the fall for unpopular decisions.
But I also had less authority to do things my way. I eventually let myself become the top banana and took the bad with the good. It had its painful moments (such as when you come in second on a new account). Ultimately it felt more honest.
My advice to you: go for it!
(Footnote: Top banana and second banana are burlesque and vaudeville terms. The second banana takes it in the kisser with the soda water. The top banana is the comic who does the spritzing.)