I think even simplistic actions can indeed be a problem. I've never had it be for me personally (yet) but even when it comes to just making a character go somewhere or walk....what if that RPer didn't want said character to go with the other? Even if you stick to the char sheets, I don't think one can assume what another character will do. You should always give the other person a chance to make their char respond how [i]they[\i] feel the character would act, not how you believe they would--even for seemingly benign actions.

Like I said before, there are ways around this. Asking the person before you make their char act, or collaborating the post with them.