Question: What barrier does Hannah Baxter remove when she acknowledges and comments directly to the audience?

Context paragraphs:
Greenhouse gas removal: Greenhouse gas removal projects are a type of climate engineering that seek to remove greenhouse gases from the atmosphere, and thus they tackle the root cause of global warming.  These techniques either directly remove greenhouse gases, or alternatively seek to influence natural processes to remove greenhouse gases indirectly.  The discipline overlaps with carbon capture and storage and carbon sequestration, and some projects listed may not be considered to be geoengineering by all commentators, instead being described as mitigation.

Hannah Montana: Live in London: Hannah Montana: Live In London was a concert held at the Koko Club on March 28, 2007 in London, England featuring Miley Cyrus as her fictional alter-ego, Hannah Montana.  She performed songs from the Hannah Montana and albums and also answered some questions from fans in the audience.  It aired on Disney XD on July 16, 2007.

Billie and the Real Belle Bare All: Billie and the Real Belle Bare All is a one-off television programme that aired on ITV2 on 25 January 2010, ahead of the Series 3 premiere of "Secret Diary of a Call Girl".  The programme, which took place at The May Fair, consisted of Billie Piper, who stars as Hannah Baxter in "Secret Diary of a Call Girl", interviewing Dr. Brooke Magnanti, a former call girl who, until November 2009 remained anonymous, known only by the pseudonym, "Belle de Jour."

James R. O'Neill (correspondent): James R. O'Neill (born February 13, 1833, Ireland; died October 6, 1863, Baxter Springs, Kansas) was a war artist and correspondent for "Frank Leslie's Illustrated Newspaper" during the American Civil War.  He covered the Battle of Honey Springs in July 1863, and his sketch of the action was published to a nationwide audience.  Less than three months later, however, O'Neill was killed in the Battle of Baxter Springs.  He is believed to be the only newsman to be killed in action during the American Civil War.

Presentational and representational acting: Presentational acting and the related representational acting are opposing ways of sustaining the actor–audience relationship.  With presentational acting, the actor acknowledges the audience.  With representational acting, the audience is studiously ignored and treated as voyeurs.

Hannah-Oke: Hannah-Oke is a British karaoke game show based on the hit TV series Hannah Montana that premiered on Disney Channel UK on 15 May 2009 and airs every week at 6pm.  It will see six families from across the UK putting their singing abilities and Hannah song knowledge to the test in a series of challenges and karaoke sing-offs in front of a studio audience and panel of judges, which consist of Disney Channel UK star Brad Kavanagh, Pop Idol participant Nicki Chapman and Dancing on Ice participant Jason Gardiner.  The audience then votes for the winning family which will go through to the final.  In the event of a tie-break, the judges vote and the family with the most votes goes through.  The three final families will then go head-to-head fir the title of Hannah-oke Champion 2009.  The winning Hannah-oke family will receive a trip to Disneyland Paris, a karaoke machine and Hannah Montana singstar game.  They will also receive a gold disk signed by Miley Cyrus.

After School Club: After School Club, often shortened to ASC, is a South Korean internet-based live-music request television talk show co-hosted by Kevin Woo, Park Jimin, and Jae.  K-pop idols Jang Han-byul and Eric Nam were the original main hosts of the show, and departed in January 2014 and April 2016, respectively.  Debuting on April 17, 2013, it is produced by Lee Hyun-ah and airs on Arirang TV.  ASC features various musical guests of South Korea and the series is directed at an international audience so the principal language is English with Korean subtitles and translations produced on air.  It allows international fans of K-pop music to interact directly with the live shows through social media: fans can join in on the live broadcasts through video chats in Google Hangouts, send in tweets to the show's official Twitter account, and are also able to share comments, ask questions, and request music videos on the show's official Facebook account.  The show is also available on Viki (website), a community for subtitles, Soompi, and on DramaFever.

Fourth wall: The fourth wall is a performance convention in which an invisible, imagined wall separates actors from the audience.  While the audience can see through this "wall", the convention assumes, the actors act as if they cannot.  From the 16th century onwards, the rise of illusionism in staging practices, which culminated in the realism and naturalism of the theatre of the 19th century, led to the development of the fourth wall concept.

Strand plain: A strand plain or strandplain is a broad belt of sand along a shoreline with a surface exhibiting well-defined parallel or semi-parallel sand ridges separated by shallow swales.  A strand plain differs from a barrier island in that it lacks either the lagoons or tidal marshes that separate a barrier island from the shoreline to which the strand plain is directly attached.  Also, the tidal channels and inlets which cut through barrier islands are absent.  Strand plains typically are created by the redistribution by waves and longshore currents of coarse sediment on either side of a river mouth.  Thus, they are part of one type of wave-dominated delta.

Hannah Baxter: Hannah Baxter is the lead semi-fictional character and protagonist of the British ITV2 television series "Secret Diary of a Call Girl".  Hannah secretly has a double life as a high-class call girl as her alter ego Belle, for which she acknowledges and comments directly to the audience, throughout each episode.

Supporting paragraphs: Hannah Baxter Fourth wall
Answer: fourth wall