Tarantino was invited to direct a scene in "Sin City" (2005) by his friend and the film's co-director, Robert Rodriguez. As Tarantino generally has no interest in the use of digital cinematography, Rodriguez encouraged him to film part of 'The Big Fat Kill' so that his friend would have a chance to experiment with the format, with real actors, and all the smorgasbord of features available on a studio production. Per Variety, Tarantino was "paid exactly $1" to direct the scene. This is not at all typical for guest directors. It may be considered typical of Tarantino and Rodriguez, as Tarantino paid Rodriguez $1 to compose the score for "Kill Bill Vol. 2" (2004).
The scene which Tarantino shot and directed would have given him the option to become involved in the framing and lighting of the scene (Rodriguez acted as Cinematographer for the entire film), and to get the performances from the two actors (Clive Owen, Benicio del Toro).
This is discussed on the second feature commentary included with the Special Edition release of the film on DVD & Blu - Ray.
IMDB's trivia section also includes the following:
The scene that Quentin Tarantino directed is the drive to the pits scene in which Dwight (Clive Owen) talks with a very dead Jackie Boy (Benicio Del Toro). When Tarantino insisted on a real car being built for the shooting Robert Rodriguez told him that it would be easier without one. After shooting a few takes with the real car, Tarantino realized his friend was right. Tarantino also came up with the idea of Jackie Boy's slit throat affecting his speech and Dwight speaking his internal monologue rather than dubbing it in later.
Robert Rodriguez scored Kill Bill: Vol. 2 (2004) for one dollar. Quentin Tarantino said he would repay him by directing a segment of this movie for one dollar. Tarantino, a vocal proponent of film-over-digital, has said that he was curious to get hands-on experience with the HD cameras which Rodriguez lauds. When asked about his experience, Tarantino merely replied, "Mission Accomplished".