I've only ever worked in local/community/amateur theatre, so I haven't been on many "formal interviews". It's more word-of-mouth for me - a "so & so knows someone who's putting together a fashion show/drag show/one-man play, etc. and needs a stage manager - would I be interested?" kinda thing. But no matter how it happens, there's always the first meeting (and first impression) with the project's director. But like I said, these meetings for me are in no way formal so I find that just looking clean, capable & low-key (clean, non-raggedy jeans, plain t-shirt or button down shirt, clean boots/sneaks, minimal jewelry, minimal baggage*) works well. If I'm asked to stay on with the project, I always find out what my dress code is, so to speak.
For most rehearsals/shows, it's always been dark & casual but well kept - black jeans that aren't faded or have holes, clean dark t-shirts without logos or holes, dark boots or sneaks that are comfortable, no flashy jewelry, long hair secured, etc.
However, I now have the unique opportunity to play dress up. At the venue where PBR performs, there is no curtain between the audience and stage. Everytime I step out there to strike a set, lay out props, or pick up clothes, I'm in full view of the crowd (it's almost 3 years and I'm still not used to it!). Therefore, I'm encouraged to get dolled up and have fun while I'm out there. And believe me - if I could move 30 lb. go-go boxes and cafeteria-sized folding tables in a corset & heels, I would! But they understand and respect my decision to go with boring old black jeans and black t-shirt.
* I once went to a first meeting/interview with my industrial-sized uber SM's bag, complete with...everything. I don't know why. I didn't need 98% of that stuff at that time anyway. As I rummaged thru the multiple pockets before the meeting, a voice behind me said "Oh great - someone else who lives outta their bag. Please tell me you're not bringing that chaos in here?" Ouch! What SM wants to be mentioned in the same sentence as "chaos"? It was the very tired, very stressed director. After the intros and apologies (me for appearing to be disorganized, her for being stressed & bitchy), I made a mental note to only bring the bare necessities, or minimal baggage, to future first meetings/interviews.