Source, para 2: The importance of Stevens’ good manners, by Prof Pamela Harris, 2010 Apr 26.
For the public, Justice Stevens' manners are perhaps most evident at oral argument. Since the Justice announced his retirement, many lawyers already have commented fondly on the Justice's habit of prefacing his remarks with a request: "May I ask you a question?" And what follows is indeed a question"” [sic for these two quotation marks] an exceptionally hard question, usually, but an actual question, in search of an actual response, so that the advocate becomes a meaningful part of the process rather than a foil for a Justice's
rhetorical gambit. In substance as well as form, Justice Stevens' style of questioning accords real respect to the lawyers who appear before him, treating them as valued participants with something important to offer. No wonder that so many of them speak so highly of the Justice.
I can infer that foil here is used figuratively and am guessing that it means a static sounding board or passive tool, which is how a Justice, who cares only about blazoning a
rhetoric gambit, would treat an advocate? Yet which definition applies?