meaning in context - What does the noun 'foil' mean, in 'a foil for a Justice's rhetorical gambit'?

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?

Answer

A "foil" is something or someone whose appearance contrasts with that of something or someone else, the better to show the other thing or person off.

The "foil" is a metaphor often used when describing the role of a character in a drama. A tired example: a bungling, good-natured pal might be the sidekick of a slick casanova, to highlight the lover's smooth-talking ways and make him seem all the more accomplished in that domain.

So we have the expressions "a mere foil" or "merely a foil", which describe characters whose role is simply to make another character look better or to highlight that other character's personality traits.

Judge Stevens didn't ask a question of the lawyer just so the judge could then proceed to say what he wanted to say, using the lawyer as a way to broach a subject (a "rhetorical gambit") and present his own views on it; rather he was actually interested in what the lawyer had to say.