Subtly. Selectively. Smartly. (Love how it's okay to use -ly words here, teehee...) Don't ever forget that your audience is pretty darn smart!! Give 'em a chance to follow your breadcrumbs, and to dissect and figure things out from what your characters do supply. Don't be "obvious"! If it feels like a character is talking to the audience versus someone else in the scene, you're either being really cute .. or really awful. Either way, ugh. Don't -- do not, do not, do not -- lean on Voice Over where it's simply one character yapping as if to him/herself or in his/her own head!! Takes a masterful and consistent hand/voice to craft good V.O. -- I try to steer clear myself. Oh, and here's an easy "do not" -- do not ever have a line of dialogue begin with: "As you know....." Occasionally, one might use a "dummy" character who knows as little about what's going on as perhaps the reading/viewing audience does at a certain point -- that dummy can serve as the recipient of catch-up (expository) knowledge via dialogue. The character needs to make sense, though -- can't just be stuck in willy nilly when an explanation is thought needed. One of the best tricks I've heard is to ask yourself: "What does the audience know right now? What do they need to know right now? Am I sure about that?" Chances are your characters have laid a trail of breadcrumbs already and the audience is already chewing away .. and the last thing you need to do is beat 'em over the head with info. Another trick is asking yourself "Is this what I [the writer] want to say? Or is it what the character wants to say?" If you steer clear of the former and rely solely on the latter, your chances of dialogue success are much greater.