My first piece of advice is to keep your ears open and your mouth shut. New stage managers often have a directorial urge, and try to suggest interpretation or artistic choices several times in rehearsal. It's been my experience that behavior like that can't help and often hinders the process.
Occasionally, a stage manager with lots of experience and a strong level of trust with the director can chime in during a sticky problem in rehearsal, but it's usually best not to.
I spent the first few years of my career insisting I was a manager, not an artist. Mostly I was convincing myself to leave my acting background behind, and to stick to the SM job description.