I've never used a shotgun for that, but I have used a mid-side stereo mic up on a boom. You can then tighten the pickup in post as desired. For the most part, you just need to remember that the closer you can get the mic to the people speaking, the better your sound will get. Keep it just out of the shot.
Also, have some lavs (preferably wireless) as backups for when you just can't deal with the boom. Case in point, on one shoot, I was out standing in a field with a car battery to power halogen driving lights (with diffusion paper in front of them in a homemade wooden holder mounted to my tripod) and wireless microphone receivers so that we could get footage of actors riding horses at dusk. Those wireless lavs, in spite of only being able to use one at a time because the receivers interfered with each other when in close proximity, were a lifesaver.
Also, test any wireless lavs out of the field before you go into the field. Lesson learned.