Devil's advocate

In common parlance, a devil's advocate is someone who takes a position for the sake of argument. This practice is generally an instructional technique in which one person argues a position that another is less familiar with, thereby teaching proper argument.

The phrase "let me play devil's advocate" or equivalent is used in group discussions to counter groupthink; the speaker is about to say something counter to the perceived group consensus and does not want to be personally ostracized for this. An example is Adlai Stevenson's prefatory remark "Someone in this group has to be a coward" before making an unpopular case for conciliation and negotiation in the movie Thirteen Days.