(NB: Thursday is "Talk Like Shakespeare Day." So start putting iambs into your pentameter now, my friends.)
Because I like to write dialogue so much, remembering to anchor it in my story is sometimes difficult for me. Especially in early drafts, I tend to have a lot of floating conversations -- the kind where you just have people talking to each other, with no clues as to where the conversation is being held, what the people are doing as they speak, and sometimes even no tags other than "she said" to indicate who is speaking.
What I realized last night, is that I am much more likely to have these floating conversations when I don't quite have the emotions of the scene right. When I know what people are feeling in the scene, I know if one person touches the other's hand while she's speaking to him, or instead turns away and fiddles with the cord of the blinds, or picks up her sword. Until I know what the characters want, I don't know if the floor is deep red tiles, cold under bare feet, or worn wood, covered with a Persian carpet with missing fringe. Every part of the story builds out from the characters.