J.L.
I'm so surprised I'm the first to answer this. My step-dad took a back seat to my mom, but there were times he put his foot down. My mom and he would have a discussion and came up with a solution together. I really want to emphasize that I think that this was VERY important. I keep reading that all the parenting/discipline needs to be done by the bio parent but I'm not sure I agree. Though I didn't love it at the time, it also made me respect him. He was setting boundaries and showed he really cared about me. I guess this depends on what the discipline looks like, but for me, it was apparent the discipline was out of love.
My step also encouraged a relationship with my dad, joked around with me a lot (in a great way), spoke about his world view (which informs how I now view politics and social issues) and really loved me. But it took a lot of time.
I really think roles emerge depending on the child's age and who the person is. When I was younger, I wouldn't dream of going to have a talk with my step-dad. Things really changed high school and esp in college. He was my "go to" when my mom had a tough time letting me make adult decisions. I do think he was great in supporting my mom in the way she chose to raise me... no tension there. What it really came down to for me was this guy was/is crazy about my mom (26 years later), treated her with respect and humor and that reflected how their relationship worked and what role he played in my life. Super vague, but that's what I have to offer.